diff --git a/browser/about_flags.cc b/browser/about_flags.cc
index 7a4b6fc57e27..e9d869d5b269 100644
--- a/browser/about_flags.cc
+++ b/browser/about_flags.cc
@@ -361,16 +361,24 @@
 #endif  // BUILDFLAG(IS_ANDROID)
 
 #if !BUILDFLAG(IS_ANDROID)
-#define BRAVE_SHARED_PINNED_TABS                                  \
-  EXPAND_FEATURE_ENTRIES({                                        \
-      "brave-shared-pinned-tabs",                                 \
-      "Shared pinned tab",                                        \
-      "Pinned tabs are shared across windows",                    \
-      kOsWin | kOsMac | kOsLinux,                                 \
-      FEATURE_VALUE_TYPE(tabs::features::kBraveSharedPinnedTabs), \
-  })
+#define BRAVE_TABS_FEATURE_ENTRIES                                        \
+  EXPAND_FEATURE_ENTRIES(                                                 \
+      {                                                                   \
+          "brave-shared-pinned-tabs",                                     \
+          "Shared pinned tab",                                            \
+          "Pinned tabs are shared across windows",                        \
+          kOsWin | kOsMac | kOsLinux,                                     \
+          FEATURE_VALUE_TYPE(tabs::features::kBraveSharedPinnedTabs),     \
+      },                                                                  \
+      {                                                                   \
+          "brave-horizontal-tabs-update",                                 \
+          "Updated horizontal tabs design",                               \
+          "Updates the look and feel or horizontal tabs",                 \
+          kOsWin | kOsMac | kOsLinux,                                     \
+          FEATURE_VALUE_TYPE(tabs::features::kBraveHorizontalTabsUpdate), \
+      })
 #else
-#define BRAVE_SHARED_PINNED_TABS
+#define BRAVE_TABS_FEATURE_ENTRIES
 #endif
 
 #if BUILDFLAG(ENABLE_AI_CHAT)
@@ -887,7 +895,7 @@
   BRAVE_BACKGROUND_VIDEO_PLAYBACK_ANDROID                                      \
   BRAVE_SAFE_BROWSING_ANDROID                                                  \
   BRAVE_CHANGE_ACTIVE_TAB_ON_SCROLL_EVENT_FEATURE_ENTRIES                      \
-  BRAVE_SHARED_PINNED_TABS                                                     \
+  BRAVE_TABS_FEATURE_ENTRIES                                                   \
   BRAVE_AI_CHAT                                                                \
   BRAVE_AI_CHAT_HISTORY                                                        \
   LAST_BRAVE_FEATURE_ENTRIES_ITEM  // Keep it as the last item.
diff --git a/browser/ui/brave_layout_constants.cc b/browser/ui/brave_layout_constants.cc
index e4c88b495b59..5ff10143c759 100644
--- a/browser/ui/brave_layout_constants.cc
+++ b/browser/ui/brave_layout_constants.cc
@@ -5,9 +5,12 @@
 
 #include "brave/browser/ui/brave_layout_constants.h"
 
+#include "brave/browser/ui/tabs/features.h"
 #include "chrome/browser/ui/layout_constants.h"
 #include "ui/base/pointer/touch_ui_controller.h"
 
+using tabs::features::HorizontalTabsUpdateEnabled;
+
 // Returns a |nullopt| if the UI color is not handled by Brave.
 absl::optional<int> GetBraveLayoutConstant(LayoutConstant constant) {
   const bool touch = ui::TouchUiController::Get()->touch_ui();
@@ -18,7 +21,15 @@ absl::optional<int> GetBraveLayoutConstant(LayoutConstant constant) {
   //     ui::MaterialDesignController::IsNewerMaterialUi();
   switch (constant) {
     case TAB_HEIGHT: {
-      return (touch ? 41 : 30) + GetLayoutConstant(TABSTRIP_TOOLBAR_OVERLAP);
+      const int tab_height = HorizontalTabsUpdateEnabled() ? 36 : 30;
+      return (touch ? 41 : tab_height) +
+             GetLayoutConstant(TABSTRIP_TOOLBAR_OVERLAP);
+    }
+    case TABSTRIP_TOOLBAR_OVERLAP: {
+      if (!HorizontalTabsUpdateEnabled()) {
+        return absl::nullopt;
+      }
+      return 0;
     }
     case TAB_SEPARATOR_HEIGHT: {
       return 24;
diff --git a/browser/ui/color/brave_color_mixer.cc b/browser/ui/color/brave_color_mixer.cc
index 0c60bcc2d725..6573338db8e2 100644
--- a/browser/ui/color/brave_color_mixer.cc
+++ b/browser/ui/color/brave_color_mixer.cc
@@ -185,6 +185,8 @@ void AddChromeLightThemeColorMixer(ui::ColorProvider* provider,
   mixer[kColorTabForegroundActiveFrameActive] = {kLightToolbarIcon};
   mixer[kColorTabForegroundInactiveFrameActive] = {
       kColorTabForegroundActiveFrameActive};
+  mixer[kColorTabStrokeFrameActive] = {SkColorSetA(SK_ColorBLACK, 0.07 * 255)};
+  mixer[kColorTabStrokeFrameInactive] = {kColorTabStrokeFrameActive};
   mixer[kColorToolbar] = {kLightToolbar};
   mixer[kColorToolbarButtonIcon] = {kColorTabForegroundActiveFrameActive};
   mixer[kColorToolbarButtonIconInactive] = {
diff --git a/browser/ui/tabs/BUILD.gn b/browser/ui/tabs/BUILD.gn
index eab12d66faf2..bae672d87615 100644
--- a/browser/ui/tabs/BUILD.gn
+++ b/browser/ui/tabs/BUILD.gn
@@ -12,12 +12,14 @@ source_set("tabs") {
 
   if (!is_android) {
     sources = [
+      "brave_tab_layout_constants.h",
       "brave_tab_menu_model.cc",
       "brave_tab_menu_model.h",
       "brave_tab_prefs.cc",
       "brave_tab_prefs.h",
       "brave_tab_strip_model.cc",
       "brave_tab_strip_model.h",
+      "brave_tab_style.h",
       "brave_vertical_tab_color_mixer.cc",
       "brave_vertical_tab_color_mixer.h",
       "features.cc",
diff --git a/browser/ui/tabs/brave_tab_layout_constants.h b/browser/ui/tabs/brave_tab_layout_constants.h
new file mode 100644
index 000000000000..fc625c716a28
--- /dev/null
+++ b/browser/ui/tabs/brave_tab_layout_constants.h
@@ -0,0 +1,54 @@
+/* Copyright (c) 2023 The Brave Authors. All rights reserved.
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+#ifndef BRAVE_BROWSER_UI_TABS_BRAVE_TAB_LAYOUT_CONSTANTS_H_
+#define BRAVE_BROWSER_UI_TABS_BRAVE_TAB_LAYOUT_CONSTANTS_H_
+
+namespace brave_tabs {
+
+// Horizontal tab layout:
+//
+// The upstream tab implemenation assumes that tab view bounds overlap. In order
+// to create a gap between tabs without violating these assumptions, tabs views
+// are given a small overlap. Rounded tab rectangles are drawn centered and
+// inset horizontally by an amount that will create the required visual gap.
+
+// The amount of space before the first tab.
+constexpr int kHorizontalTabStripLeftMargin = 8;
+
+// The amount of vertical spacing between the top and bottom of tabs and the
+// bounds of the tab strip region. The portion of this space below tabs will be
+// occupied by tab group underlines.
+constexpr int kHorizontalTabStripVerticalSpacing = 4;
+
+// The visual gap between tabs.
+constexpr int kHorizontalTabGap = 4;
+
+// The amount of overlap between tabs. Based on upstream assumptions, tab views
+// must have a non-negative overlap. Furthermore, tab separators will not render
+// correctly if the tab overlap is zero.
+constexpr int kHorizontalTabOverlap = 2;
+
+// The horizontal difference between the edge of the tab view and the visual
+// edge of the rendered tab.
+constexpr int kHorizontalTabInset =
+    (kHorizontalTabGap + kHorizontalTabOverlap) / 2;
+
+// The content padding within a tab.
+constexpr int kHorizontalTabPadding = 6;
+
+// The horizontal difference between the visual edge of a tab group and the
+// bounds of the group underline.
+constexpr int kHorizontalGroupUnderlineInset = 2;
+
+// The tab border radius.
+constexpr int kTabBorderRadius = 8;
+
+// The size of the group header slot when the title is empty.
+constexpr int kEmptyGroupTitleSize = 22;
+
+}  // namespace brave_tabs
+
+#endif  // BRAVE_BROWSER_UI_TABS_BRAVE_TAB_LAYOUT_CONSTANTS_H_
diff --git a/browser/ui/tabs/brave_tab_style.h b/browser/ui/tabs/brave_tab_style.h
new file mode 100644
index 000000000000..34783f946f13
--- /dev/null
+++ b/browser/ui/tabs/brave_tab_style.h
@@ -0,0 +1,57 @@
+/* Copyright (c) 2023 The Brave Authors. All rights reserved.
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+#ifndef BRAVE_BROWSER_UI_TABS_BRAVE_TAB_STYLE_H_
+#define BRAVE_BROWSER_UI_TABS_BRAVE_TAB_STYLE_H_
+
+#include "brave/browser/ui/tabs/brave_tab_layout_constants.h"
+#include "brave/browser/ui/tabs/features.h"
+#include "chrome/browser/ui/layout_constants.h"
+#include "ui/gfx/geometry/insets.h"
+
+// A subclass of TabStyle used to customize tab layout and visuals. It is
+// implemented as a template because it must be included in the source file
+// override before the base class definition.
+template <typename TabStyleBase>
+class BraveTabStyle : public TabStyleBase {
+ public:
+  int GetTabOverlap() const override {
+    if (!tabs::features::HorizontalTabsUpdateEnabled()) {
+      return TabStyleBase::GetTabOverlap();
+    }
+    return brave_tabs::kHorizontalTabOverlap;
+  }
+
+  int GetTopCornerRadius() const override {
+    if (!tabs::features::HorizontalTabsUpdateEnabled()) {
+      return TabStyleBase::GetTopCornerRadius();
+    }
+    return brave_tabs::kTabBorderRadius;
+  }
+
+  int GetBottomCornerRadius() const override {
+    if (!tabs::features::HorizontalTabsUpdateEnabled()) {
+      return TabStyleBase::GetBottomCornerRadius();
+    }
+    return brave_tabs::kTabBorderRadius;
+  }
+
+  gfx::Insets GetContentsInsets() const override {
+    if (!tabs::features::HorizontalTabsUpdateEnabled()) {
+      return TabStyleBase::GetContentsInsets();
+    }
+    return gfx::Insets::VH(
+        0, brave_tabs::kHorizontalTabPadding + brave_tabs::kHorizontalTabInset);
+  }
+
+  int GetPinnedWidth() const override {
+    if (!tabs::features::HorizontalTabsUpdateEnabled()) {
+      return TabStyleBase::GetPinnedWidth();
+    }
+    return GetLayoutConstant(TAB_HEIGHT) + brave_tabs::kHorizontalTabInset * 2;
+  }
+};
+
+#endif  // BRAVE_BROWSER_UI_TABS_BRAVE_TAB_STYLE_H_
diff --git a/browser/ui/tabs/features.cc b/browser/ui/tabs/features.cc
index 70cbd8c613d1..c1c543967648 100644
--- a/browser/ui/tabs/features.cc
+++ b/browser/ui/tabs/features.cc
@@ -17,4 +17,12 @@ BASE_FEATURE(kBraveSharedPinnedTabs,
              "BraveSharedPinnedTabs",
              base::FEATURE_DISABLED_BY_DEFAULT);
 
+BASE_FEATURE(kBraveHorizontalTabsUpdate,
+             "BraveHorizontalTabsUpdate",
+             base::FEATURE_DISABLED_BY_DEFAULT);
+
+bool HorizontalTabsUpdateEnabled() {
+  return base::FeatureList::IsEnabled(kBraveHorizontalTabsUpdate);
+}
+
 }  // namespace tabs::features
diff --git a/browser/ui/tabs/features.h b/browser/ui/tabs/features.h
index f3708f4e66b3..4665294bf487 100644
--- a/browser/ui/tabs/features.h
+++ b/browser/ui/tabs/features.h
@@ -17,6 +17,10 @@ BASE_DECLARE_FEATURE(kBraveChangeActiveTabOnScrollEvent);
 
 BASE_DECLARE_FEATURE(kBraveSharedPinnedTabs);
 
+BASE_DECLARE_FEATURE(kBraveHorizontalTabsUpdate);
+
+bool HorizontalTabsUpdateEnabled();
+
 }  // namespace tabs::features
 
 #endif  // BRAVE_BROWSER_UI_TABS_FEATURES_H_
diff --git a/browser/ui/views/frame/brave_browser_non_client_frame_view_mac.mm b/browser/ui/views/frame/brave_browser_non_client_frame_view_mac.mm
index 7b20d4adacf2..56e4e684001a 100644
--- a/browser/ui/views/frame/brave_browser_non_client_frame_view_mac.mm
+++ b/browser/ui/views/frame/brave_browser_non_client_frame_view_mac.mm
@@ -8,6 +8,7 @@
 #include "brave/browser/ui/views/frame/brave_browser_non_client_frame_view_mac.h"
 
 #include "brave/browser/ui/tabs/brave_tab_prefs.h"
+#include "brave/browser/ui/tabs/features.h"
 #include "brave/browser/ui/views/frame/brave_non_client_hit_test_helper.h"
 #include "brave/browser/ui/views/frame/brave_window_frame_graphic.h"
 #include "brave/browser/ui/views/tabs/vertical_tab_utils.h"
@@ -66,7 +67,13 @@
     return 30;
   }
 
-  return BrowserNonClientFrameViewMac::GetTopInset(restored);
+  if (!tabs::features::HorizontalTabsUpdateEnabled()) {
+    return BrowserNonClientFrameViewMac::GetTopInset(restored);
+  }
+
+  // The tab region view maintains its own padding, but insert a small gap to
+  // give a bit more room for the frame resize handle.
+  return 2;
 }
 
 bool BraveBrowserNonClientFrameViewMac::ShouldShowWindowTitleForVerticalTabs()
diff --git a/browser/ui/views/frame/vertical_tab_strip_region_view.cc b/browser/ui/views/frame/vertical_tab_strip_region_view.cc
index 83711347622d..13a1956e74b4 100644
--- a/browser/ui/views/frame/vertical_tab_strip_region_view.cc
+++ b/browser/ui/views/frame/vertical_tab_strip_region_view.cc
@@ -309,7 +309,7 @@ class VerticalTabNewTabButton : public BraveNewTabButton {
                        float scale,
                        bool extend_to_top) const override {
     auto contents_bounds = GetContentsBounds();
-    const float radius = tabs::kUnpinnedTabBorderRadius * scale;
+    const float radius = GetCornerRadius() * scale;
     SkPath path;
     const gfx::Rect path_rect(origin.x(), origin.y(),
                               contents_bounds.width() * scale,
diff --git a/browser/ui/views/tabs/brave_compound_tab_container.cc b/browser/ui/views/tabs/brave_compound_tab_container.cc
index 4dc735227daf..3aa9e9dbf1a0 100644
--- a/browser/ui/views/tabs/brave_compound_tab_container.cc
+++ b/browser/ui/views/tabs/brave_compound_tab_container.cc
@@ -138,6 +138,7 @@ void BraveCompoundTabContainer::SetScrollEnabled(bool enabled) {
 
   if (enabled) {
     scroll_view_ = AddChildView(std::make_unique<CustomScrollView>());
+    scroll_view_->SetBackgroundThemeColorId(kColorToolbar);
     auto* contents_view =
         scroll_view_->SetContents(std::make_unique<ContentsView>(this));
     contents_view->AddChildView(base::to_address(unpinned_tab_container_));
diff --git a/browser/ui/views/tabs/brave_new_tab_button.cc b/browser/ui/views/tabs/brave_new_tab_button.cc
index 6a5131547e61..adc9144f3202 100644
--- a/browser/ui/views/tabs/brave_new_tab_button.cc
+++ b/browser/ui/views/tabs/brave_new_tab_button.cc
@@ -6,19 +6,26 @@
 #include "brave/browser/ui/views/tabs/brave_new_tab_button.h"
 
 #include <algorithm>
-#include <memory>
 #include <utility>
 
-#include "brave/browser/ui/color/brave_color_id.h"
-#include "chrome/app/chrome_command_ids.h"
-#include "chrome/browser/ui/layout_constants.h"
+#include "brave/browser/ui/tabs/features.h"
+#include "brave/components/vector_icons/vector_icons.h"
 #include "chrome/browser/ui/views/tabs/new_tab_button.h"
 #include "chrome/browser/ui/views/tabs/tab_strip.h"
 #include "ui/gfx/geometry/skia_conversions.h"
+#include "ui/gfx/paint_vector_icon.h"
 #include "ui/gfx/scoped_canvas.h"
+#include "ui/views/view_class_properties.h"
+
+using tabs::features::HorizontalTabsUpdateEnabled;
 
 // static
-const gfx::Size BraveNewTabButton::kButtonSize{24, 24};
+gfx::Size BraveNewTabButton::GetButtonSize() {
+  if (!HorizontalTabsUpdateEnabled()) {
+    return {24, 24};
+  }
+  return {28, 28};
+}
 
 // static
 SkPath BraveNewTabButton::GetBorderPath(const gfx::Point& origin,
@@ -44,7 +51,7 @@ SkPath BraveNewTabButton::GetBorderPath(const gfx::Point& origin,
 
 gfx::Size BraveNewTabButton::CalculatePreferredSize() const {
   // Overriden so that we use Brave's custom button size
-  gfx::Size size = kButtonSize;
+  gfx::Size size = GetButtonSize();
   const auto insets = GetInsets();
   size.Enlarge(insets.width(), insets.height());
   return size;
@@ -59,12 +66,31 @@ SkPath BraveNewTabButton::GetBorderPath(const gfx::Point& origin,
 
 BraveNewTabButton::BraveNewTabButton(TabStrip* tab_strip,
                                      PressedCallback callback)
-    : NewTabButton(tab_strip, std::move(callback)) {}
+    : NewTabButton(tab_strip, std::move(callback)) {
+  if (HorizontalTabsUpdateEnabled()) {
+    // Ensure that the new tab button is vertically centered within its flex
+    // layout container.
+    SetProperty(views::kCrossAxisAlignmentKey, views::LayoutAlignment::kCenter);
+  }
+}
 
 BraveNewTabButton::~BraveNewTabButton() = default;
 
 void BraveNewTabButton::PaintIcon(gfx::Canvas* canvas) {
   gfx::ScopedCanvas scoped_canvas(canvas);
+
+  if (HorizontalTabsUpdateEnabled()) {
+    // Instead of letting `NewTabButton` draw a "plus", paint a vector icon to
+    // the canvas in the center of the view.
+    constexpr int kIconSize = 16;
+    gfx::Rect bounds = GetContentsBounds();
+    canvas->Translate(gfx::Vector2d((bounds.width() - kIconSize) / 2,
+                                    (bounds.height() - kIconSize) / 2));
+    gfx::PaintVectorIcon(canvas, kLeoPlusAddIcon, kIconSize,
+                         GetForegroundColor());
+    return;
+  }
+
   // Shim base implementation's painting
   // Overriden to fix chromium assumption that border radius
   // will be 50% of width.
diff --git a/browser/ui/views/tabs/brave_new_tab_button.h b/browser/ui/views/tabs/brave_new_tab_button.h
index 24fccea58c31..76b7632283ff 100644
--- a/browser/ui/views/tabs/brave_new_tab_button.h
+++ b/browser/ui/views/tabs/brave_new_tab_button.h
@@ -21,15 +21,14 @@ class BraveNewTabButton : public NewTabButton {
   // These static members are shared with BraveTabSearchButton
   // TODO(sko) If we could make TabSearchButton inherit BraveNewTabButton,
   // we might not need these any more.
-  static const gfx::Size kButtonSize;
+  static gfx::Size GetButtonSize();
   static SkPath GetBorderPath(const gfx::Point& origin,
                               float scale,
                               bool extend_to_top,
                               int border_radius,
                               const gfx::Size& contents_bounds);
+
   BraveNewTabButton(TabStrip* tab_strip, PressedCallback callback);
-  BraveNewTabButton(const BraveNewTabButton&) = delete;
-  BraveNewTabButton& operator=(const BraveNewTabButton&) = delete;
   ~BraveNewTabButton() override;
 
  protected:
diff --git a/browser/ui/views/tabs/brave_tab.cc b/browser/ui/views/tabs/brave_tab.cc
index 7388604f3911..416bb1a99027 100644
--- a/browser/ui/views/tabs/brave_tab.cc
+++ b/browser/ui/views/tabs/brave_tab.cc
@@ -8,6 +8,7 @@
 #include <algorithm>
 
 #include "brave/browser/ui/tabs/brave_tab_prefs.h"
+#include "brave/browser/ui/tabs/features.h"
 #include "brave/browser/ui/views/frame/brave_browser_view.h"
 #include "brave/browser/ui/views/frame/vertical_tab_strip_region_view.h"
 #include "brave/browser/ui/views/frame/vertical_tab_strip_widget_delegate_view.h"
@@ -90,10 +91,8 @@ class ShadowLayer : public ui::Layer, public ui::LayerDelegate {
     // bounds.
     gfx::Rect shadow_bounds(size());
     shadow_bounds.Inset(GetBlurRegionInsets());
-    const int kCornerRadius =
-        (tab_->data().pinned ? tabs::kPinnedTabBorderRadius
-                             : tabs::kUnpinnedTabBorderRadius);
-    recorder.canvas()->DrawRoundRect(shadow_bounds, kCornerRadius, flags);
+    const int radius = tabs::GetTabCornerRadius(*tab_);
+    recorder.canvas()->DrawRoundRect(shadow_bounds, radius, flags);
   }
 
   void OnDeviceScaleFactorChanged(float old_device_scale_factor,
@@ -160,7 +159,12 @@ absl::optional<SkColor> BraveTab::GetGroupColor() const {
     return {};
   }
 
-  return Tab::GetGroupColor();
+  if (!tabs::features::HorizontalTabsUpdateEnabled()) {
+    return Tab::GetGroupColor();
+  }
+
+  // Unlike upstream, tabs that are within a group are not given a border color.
+  return {};
 }
 
 void BraveTab::UpdateIconVisibility() {
@@ -288,8 +292,11 @@ bool BraveTab::IsAtMinWidthForVerticalTabStrip() const {
 }
 
 void BraveTab::UpdateShadowForActiveTab() {
-  if (IsActive() &&
-      tabs::utils::ShouldShowVerticalTabs(controller()->GetBrowser())) {
+  bool can_render_shadows =
+      tabs::features::HorizontalTabsUpdateEnabled() ||
+      tabs::utils::ShouldShowVerticalTabs(controller()->GetBrowser());
+
+  if (IsActive() && can_render_shadows) {
     shadow_layer_ = CreateShadowLayer();
     AddLayerToBelowThis();
     LayoutShadowLayer();
diff --git a/browser/ui/views/tabs/brave_tab_container.cc b/browser/ui/views/tabs/brave_tab_container.cc
index 9ad4b399f548..1ec78239ba8d 100644
--- a/browser/ui/views/tabs/brave_tab_container.cc
+++ b/browser/ui/views/tabs/brave_tab_container.cc
@@ -11,6 +11,7 @@
 
 #include "base/check_is_test.h"
 #include "base/containers/flat_map.h"
+#include "brave/browser/ui/tabs/brave_tab_layout_constants.h"
 #include "brave/browser/ui/tabs/brave_tab_prefs.h"
 #include "brave/browser/ui/tabs/features.h"
 #include "brave/browser/ui/views/frame/brave_browser_view.h"
@@ -32,6 +33,20 @@
 #include "ui/gfx/skbitmap_operations.h"
 #include "ui/views/view_utils.h"
 
+namespace {
+
+gfx::Size AddHorizontalTabStripSpacing(gfx::Size size) {
+  if (!tabs::features::HorizontalTabsUpdateEnabled()) {
+    return size;
+  }
+  // Allow for a small space at the top and bottom of the tab strip. Tab group
+  // underlines will partially occupy the space below tabs.
+  size.Enlarge(0, brave_tabs::kHorizontalTabStripVerticalSpacing * 2);
+  return size;
+}
+
+}  // namespace
+
 BraveTabContainer::BraveTabContainer(
     TabContainerController& controller,
     TabHoverCardController* hover_card_controller,
@@ -91,6 +106,14 @@ base::OnceClosure BraveTabContainer::LockLayout() {
                         base::Unretained(this));
 }
 
+gfx::Size BraveTabContainer::GetMinimumSize() const {
+  gfx::Size size = TabContainerImpl::GetMinimumSize();
+  if (tabs::utils::ShouldShowVerticalTabs(tab_slot_controller_->GetBrowser())) {
+    return size;
+  }
+  return AddHorizontalTabStripSpacing(size);
+}
+
 gfx::Size BraveTabContainer::CalculatePreferredSize() const {
   // Note that we check this before checking currently we're in vertical tab
   // strip mode. We might be in the middle of changing orientation.
@@ -100,7 +123,8 @@ gfx::Size BraveTabContainer::CalculatePreferredSize() const {
 
   if (!tabs::utils::ShouldShowVerticalTabs(
           tab_slot_controller_->GetBrowser())) {
-    return TabContainerImpl::CalculatePreferredSize();
+    return AddHorizontalTabStripSpacing(
+        TabContainerImpl::CalculatePreferredSize());
   }
 
   const int tab_count = tabs_view_model_.view_size();
@@ -277,16 +301,6 @@ void BraveTabContainer::CompleteAnimationAndLayout() {
   base::ranges::for_each(children(), &views::View::Layout);
 }
 
-void BraveTabContainer::OnPaintBackground(gfx::Canvas* canvas) {
-  if (!tabs::utils::ShouldShowVerticalTabs(
-          tab_slot_controller_->GetBrowser())) {
-    TabContainerImpl::OnPaintBackground(canvas);
-    return;
-  }
-
-  canvas->DrawColor(GetColorProvider()->GetColor(kColorToolbar));
-}
-
 void BraveTabContainer::PaintChildren(const views::PaintInfo& paint_info) {
   // Exclude tabs that own layer.
   std::vector<ZOrderableTabContainerElement> orderable_children;
diff --git a/browser/ui/views/tabs/brave_tab_container.h b/browser/ui/views/tabs/brave_tab_container.h
index 1c4b18e31d98..655e6c943576 100644
--- a/browser/ui/views/tabs/brave_tab_container.h
+++ b/browser/ui/views/tabs/brave_tab_container.h
@@ -32,6 +32,7 @@ class BraveTabContainer : public TabContainerImpl {
   base::OnceClosure LockLayout();
 
   // TabContainerImpl:
+  gfx::Size GetMinimumSize() const override;
   gfx::Size CalculatePreferredSize() const override;
   void UpdateClosingModeOnRemovedTab(int model_index, bool was_active) override;
   gfx::Rect GetTargetBoundsForClosingTab(Tab* tab,
@@ -43,7 +44,6 @@ class BraveTabContainer : public TabContainerImpl {
   void RemoveTab(int index, bool was_active) override;
   void OnTabCloseAnimationCompleted(Tab* tab) override;
   void CompleteAnimationAndLayout() override;
-  void OnPaintBackground(gfx::Canvas* canvas) override;
   void PaintChildren(const views::PaintInfo& paint_info) override;
 
   // BrowserRootView::DropTarget
diff --git a/browser/ui/views/tabs/brave_tab_group_header.cc b/browser/ui/views/tabs/brave_tab_group_header.cc
index 93ceb84f6fb2..e6e4a6b364f9 100644
--- a/browser/ui/views/tabs/brave_tab_group_header.cc
+++ b/browser/ui/views/tabs/brave_tab_group_header.cc
@@ -5,6 +5,7 @@
 
 #include "brave/browser/ui/views/tabs/brave_tab_group_header.h"
 
+#include "brave/browser/ui/tabs/features.h"
 #include "brave/browser/ui/views/tabs/vertical_tab_utils.h"
 #include "chrome/browser/ui/browser.h"
 #include "chrome/browser/ui/tabs/tab_group_model.h"
@@ -20,9 +21,8 @@
 
 namespace {
 
-SkColor GetGroupBackgroundColorForVerticalTabs(
-    const tab_groups::TabGroupId& group_id,
-    TabSlotController* controller) {
+SkColor GetGroupBackgroundColor(const tab_groups::TabGroupId& group_id,
+                                TabSlotController* controller) {
   if (!controller->GetBrowser()
            ->tab_strip_model()
            ->group_model()
@@ -60,11 +60,13 @@ void BraveTabGroupHeader::AddedToWidget() {
 
 void BraveTabGroupHeader::VisualsChanged() {
   TabGroupHeader::VisualsChanged();
-  if (!ShouldShowVerticalTabs()) {
+
+  if (!tabs::features::HorizontalTabsUpdateEnabled() &&
+      !ShouldShowVerticalTabs()) {
     return;
   }
 
-  title_->SetEnabledColor(GetGroupBackgroundColorForVerticalTabs(
+  title_->SetEnabledColor(GetGroupBackgroundColor(
       group().value(), base::to_address(tab_slot_controller_)));
   title_->SetSubpixelRenderingEnabled(false);
 
@@ -72,19 +74,18 @@ void BraveTabGroupHeader::VisualsChanged() {
   title_->SetFontList(font_list.DeriveWithWeight(gfx::Font::Weight::MEDIUM)
                           .DeriveWithSizeDelta(13 - font_list.GetFontSize()));
 
-  // We don't draw background for vertical tabs.
   title_chip_->SetBackground(nullptr);
 
-  LayoutTitleChip();
+  if (ShouldShowVerticalTabs()) {
+    LayoutTitleChipForVerticalTabs();
+  }
 }
 
 void BraveTabGroupHeader::Layout() {
   TabGroupHeader::Layout();
-  if (!ShouldShowVerticalTabs()) {
-    return;
+  if (ShouldShowVerticalTabs()) {
+    LayoutTitleChipForVerticalTabs();
   }
-
-  LayoutTitleChip();
 }
 
 bool BraveTabGroupHeader::ShouldShowVerticalTabs() const {
@@ -92,7 +93,7 @@ bool BraveTabGroupHeader::ShouldShowVerticalTabs() const {
       tab_slot_controller_->GetBrowser());
 }
 
-void BraveTabGroupHeader::LayoutTitleChip() {
+void BraveTabGroupHeader::LayoutTitleChipForVerticalTabs() {
   auto title_bounds = GetContentsBounds();
   title_bounds.Inset(gfx::Insets(kPaddingForGroup * 2));
   title_chip_->SetBoundsRect(title_bounds);
diff --git a/browser/ui/views/tabs/brave_tab_group_header.h b/browser/ui/views/tabs/brave_tab_group_header.h
index 5ff10675106c..00635389ac95 100644
--- a/browser/ui/views/tabs/brave_tab_group_header.h
+++ b/browser/ui/views/tabs/brave_tab_group_header.h
@@ -28,7 +28,7 @@ class BraveTabGroupHeader : public TabGroupHeader {
 
  private:
   bool ShouldShowVerticalTabs() const;
-  void LayoutTitleChip();
+  void LayoutTitleChipForVerticalTabs();
 };
 
 #endif  // BRAVE_BROWSER_UI_VIEWS_TABS_BRAVE_TAB_GROUP_HEADER_H_
diff --git a/browser/ui/views/tabs/brave_tab_group_highlight.cc b/browser/ui/views/tabs/brave_tab_group_highlight.cc
index 9a620817e990..8cd2e33c20ca 100644
--- a/browser/ui/views/tabs/brave_tab_group_highlight.cc
+++ b/browser/ui/views/tabs/brave_tab_group_highlight.cc
@@ -5,6 +5,8 @@
 
 #include "brave/browser/ui/views/tabs/brave_tab_group_highlight.h"
 
+#include "brave/browser/ui/tabs/brave_tab_layout_constants.h"
+#include "brave/browser/ui/tabs/features.h"
 #include "brave/browser/ui/views/tabs/brave_tab_group_header.h"
 #include "brave/browser/ui/views/tabs/vertical_tab_utils.h"
 #include "chrome/browser/ui/views/tabs/tab_group_views.h"
@@ -12,10 +14,24 @@
 BraveTabGroupHighlight::~BraveTabGroupHighlight() = default;
 
 SkPath BraveTabGroupHighlight::GetPath() const {
-  if (!tabs::utils::ShouldShowVerticalTabs(tab_group_views_->GetBrowser())) {
+  // We don't have to paint a highlight for vertical tabs.
+  if (tabs::utils::ShouldShowVerticalTabs(tab_group_views_->GetBrowser())) {
+    return {};
+  }
+
+  if (!tabs::features::HorizontalTabsUpdateEnabled()) {
     return TabGroupHighlight::GetPath();
   }
 
-  // We don't have to paint highlight for vertical tabs
-  return {};
+  // Draw a rounded rect that encloses the header and all tabs within the
+  // group.
+  float tab_top = 0;
+  float tab_left = brave_tabs::kHorizontalTabInset;
+  float tab_right = bounds().width() - brave_tabs::kHorizontalTabInset;
+  float tab_bottom = bounds().height();
+  float radius = brave_tabs::kTabBorderRadius;
+
+  SkPath path;
+  path.addRoundRect({tab_left, tab_top, tab_right, tab_bottom}, radius, radius);
+  return path;
 }
diff --git a/browser/ui/views/tabs/brave_tab_group_underline.cc b/browser/ui/views/tabs/brave_tab_group_underline.cc
index 35a992d79344..cf4061f7fac5 100644
--- a/browser/ui/views/tabs/brave_tab_group_underline.cc
+++ b/browser/ui/views/tabs/brave_tab_group_underline.cc
@@ -7,6 +7,8 @@
 
 #include <algorithm>
 
+#include "brave/browser/ui/tabs/brave_tab_layout_constants.h"
+#include "brave/browser/ui/tabs/features.h"
 #include "brave/browser/ui/views/tabs/brave_tab_group_header.h"
 #include "brave/browser/ui/views/tabs/vertical_tab_utils.h"
 #include "cc/paint/paint_flags.h"
@@ -14,6 +16,7 @@
 #include "chrome/browser/ui/views/tabs/tab_group_views.h"
 #include "ui/gfx/canvas.h"
 #include "ui/gfx/geometry/skia_conversions.h"
+#include "ui/views/view_utils.h"
 
 BraveTabGroupUnderline::BraveTabGroupUnderline(
     TabGroupViews* tab_group_views,
@@ -44,10 +47,21 @@ void BraveTabGroupUnderline::UpdateBounds(const views::View* leading_view,
 
 gfx::Insets BraveTabGroupUnderline::GetInsetsForUnderline(
     const views::View* sibling_view) const {
-  if (!ShouldShowVerticalTabs())
+  if (ShouldShowVerticalTabs()) {
+    return {};
+  }
+
+  if (!tabs::features::HorizontalTabsUpdateEnabled()) {
     return TabGroupUnderline::GetInsetsForUnderline(sibling_view);
+  }
 
-  return {};
+  // For horizontal tabs, the underline should be inset slightly within the
+  // visual edges of the tab.
+  int horizontal_inset = TabGroupUnderline::kStrokeThickness +
+                         brave_tabs::kHorizontalTabInset +
+                         brave_tabs::kHorizontalGroupUnderlineInset;
+
+  return gfx::Insets::VH(0, horizontal_inset);
 }
 
 gfx::Rect BraveTabGroupUnderline::CalculateTabGroupUnderlineBounds(
@@ -55,8 +69,17 @@ gfx::Rect BraveTabGroupUnderline::CalculateTabGroupUnderlineBounds(
     const views::View* const leading_view,
     const views::View* const trailing_view) const {
   if (!ShouldShowVerticalTabs()) {
-    return TabGroupUnderline::CalculateTabGroupUnderlineBounds(
+    auto bounds = TabGroupUnderline::CalculateTabGroupUnderlineBounds(
         underline_view, leading_view, trailing_view);
+
+    if (tabs::features::HorizontalTabsUpdateEnabled()) {
+      // Upstream places the underline at the bottom tab border. Push the
+      // underline down to the bottom of the tab strip, so that it will appear
+      // below the tabs.
+      bounds.Offset(0, brave_tabs::kHorizontalTabStripVerticalSpacing);
+    }
+
+    return bounds;
   }
 
   // override bounds for vertical tabs mode.
diff --git a/browser/ui/views/tabs/brave_tab_search_button.cc b/browser/ui/views/tabs/brave_tab_search_button.cc
index 6189ccf89950..195a55aa5b15 100644
--- a/browser/ui/views/tabs/brave_tab_search_button.cc
+++ b/browser/ui/views/tabs/brave_tab_search_button.cc
@@ -5,11 +5,12 @@
 
 #include "brave/browser/ui/views/tabs/brave_tab_search_button.h"
 
-#include <algorithm>
 #include <memory>
 
+#include "brave/browser/ui/tabs/features.h"
 #include "brave/browser/ui/views/brave_tab_search_bubble_host.h"
 #include "brave/browser/ui/views/tabs/brave_new_tab_button.h"
+#include "brave/components/vector_icons/vector_icons.h"
 #include "chrome/browser/ui/views/chrome_layout_provider.h"
 #include "chrome/browser/ui/views/tabs/new_tab_button.h"
 #include "chrome/browser/ui/views/tabs/tab_strip_controller.h"
@@ -29,7 +30,12 @@ BraveTabSearchButton::BraveTabSearchButton(TabStrip* tab_strip)
 BraveTabSearchButton::~BraveTabSearchButton() = default;
 
 gfx::Size BraveTabSearchButton::CalculatePreferredSize() const {
-  return BraveNewTabButton::kButtonSize;
+  auto size = BraveNewTabButton::GetButtonSize();
+  if (tabs::features::HorizontalTabsUpdateEnabled()) {
+    auto insets = GetInsets();
+    size.Enlarge(insets.width(), insets.height());
+  }
+  return size;
 }
 
 void BraveTabSearchButton::SetBubbleArrow(views::BubbleBorder::Arrow arrow) {
@@ -37,6 +43,26 @@ void BraveTabSearchButton::SetBubbleArrow(views::BubbleBorder::Arrow arrow) {
       ->SetBubbleArrow(arrow);
 }
 
+void BraveTabSearchButton::UpdateColors() {
+  TabSearchButton::UpdateColors();
+
+  if (!tabs::features::HorizontalTabsUpdateEnabled()) {
+    return;
+  }
+
+  // Use a custom icon for tab search.
+  constexpr int kIconSize = 16;
+  SetImageModel(views::Button::STATE_NORMAL,
+                ui::ImageModel::FromVectorIcon(
+                    kLeoSearchIcon, GetForegroundColor(), kIconSize));
+  SetImageModel(views::Button::STATE_HOVERED, ui::ImageModel());
+  SetImageModel(views::Button::STATE_PRESSED, ui::ImageModel());
+
+  // Unset any backgrounds or borders.
+  SetBorder(nullptr);
+  SetBackground(nullptr);
+}
+
 int BraveTabSearchButton::GetCornerRadius() const {
   return ChromeLayoutProvider::Get()->GetCornerRadiusMetric(
       views::Emphasis::kMaximum, GetContentsBounds().size());
diff --git a/browser/ui/views/tabs/brave_tab_search_button.h b/browser/ui/views/tabs/brave_tab_search_button.h
index b18818d2d21e..39479abe7941 100644
--- a/browser/ui/views/tabs/brave_tab_search_button.h
+++ b/browser/ui/views/tabs/brave_tab_search_button.h
@@ -21,7 +21,8 @@ class BraveTabSearchButton : public TabSearchButton {
 
   void SetBubbleArrow(views::BubbleBorder::Arrow arrow);
 
-  // TabSearchButton overrides:
+  // TabSearchButton:
+  void UpdateColors() override;
   gfx::Size CalculatePreferredSize() const override;
   int GetCornerRadius() const override;
 };
diff --git a/browser/ui/views/tabs/brave_tab_strip.cc b/browser/ui/views/tabs/brave_tab_strip.cc
index caee32658121..8fcbe3fe1487 100644
--- a/browser/ui/views/tabs/brave_tab_strip.cc
+++ b/browser/ui/views/tabs/brave_tab_strip.cc
@@ -10,6 +10,7 @@
 #include "brave/browser/profiles/profile_util.h"
 #include "brave/browser/themes/brave_dark_mode_utils.h"
 #include "brave/browser/ui/color/brave_color_id.h"
+#include "brave/browser/ui/tabs/brave_tab_layout_constants.h"
 #include "brave/browser/ui/tabs/brave_tab_prefs.h"
 #include "brave/browser/ui/tabs/features.h"
 #include "brave/browser/ui/tabs/shared_pinned_tab_service.h"
@@ -37,10 +38,20 @@
 #include "third_party/skia/include/core/SkColor.h"
 #include "ui/base/metadata/metadata_impl_macros.h"
 #include "ui/compositor/layer.h"
+#include "ui/gfx/canvas.h"
 #include "ui/views/layout/flex_layout.h"
 
+using tabs::features::HorizontalTabsUpdateEnabled;
+
 BraveTabStrip::BraveTabStrip(std::unique_ptr<TabStripController> controller)
-    : TabStrip(std::move(controller)) {}
+    : TabStrip(std::move(controller)) {
+  if (HorizontalTabsUpdateEnabled()) {
+    // In order for active tab shadow effects to be applied consistently across
+    // platforms, an ancestor of the active tab and shadow need to be painted to
+    // a layer.
+    SetPaintToLayer();
+  }
+}
 
 BraveTabStrip::~BraveTabStrip() = default;
 
@@ -80,6 +91,12 @@ bool BraveTabStrip::ShouldDrawStrokes() const {
     return false;
   }
 
+  if (HorizontalTabsUpdateEnabled()) {
+    // We never automatically draw strokes around tabs. For pinned tabs, we draw
+    // the stroke when generating the tab drawing path.
+    return false;
+  }
+
   if (!TabStrip::ShouldDrawStrokes()) {
     return false;
   }
@@ -105,7 +122,7 @@ bool BraveTabStrip::ShouldDrawStrokes() const {
 }
 
 int BraveTabStrip::GetStrokeThickness() const {
-  if (ShouldShowVerticalTabs()) {
+  if (ShouldShowVerticalTabs() || HorizontalTabsUpdateEnabled()) {
     // Bypass checking ShouldDrawStrokes().
     return 1;
   }
@@ -163,6 +180,8 @@ void BraveTabStrip::MaybeStartDrag(
 void BraveTabStrip::AddedToWidget() {
   TabStrip::AddedToWidget();
 
+  UpdateTabStripMargins();
+
   if (BrowserView::GetBrowserViewForBrowser(GetBrowser())) {
     UpdateTabContainer();
   } else {
@@ -375,6 +394,29 @@ void BraveTabStrip::UpdateTabContainer() {
   }
 }
 
+void BraveTabStrip::UpdateTabStripMargins() {
+  if (!HorizontalTabsUpdateEnabled()) {
+    return;
+  }
+
+  gfx::Insets margins;
+
+  if (!ShouldShowVerticalTabs()) {
+    // There should be a medium size gap between the left edge of the tabstrip
+    // and the visual left edge of the first tab. Set a left margin that takes
+    // into account the visual tab inset.
+    margins.set_left(brave_tabs::kHorizontalTabStripLeftMargin -
+                     brave_tabs::kHorizontalTabInset);
+    DCHECK_GE(margins.left(), 0);
+
+    // Set a top margin to match the space under tabs (where the group underline
+    // is rendered), so that everything remains centered.
+    margins.set_top(brave_tabs::kHorizontalTabStripVerticalSpacing);
+  }
+
+  SetProperty(views::kMarginsKey, margins);
+}
+
 bool BraveTabStrip::ShouldShowVerticalTabs() const {
   return tabs::utils::ShouldShowVerticalTabs(GetBrowser());
 }
@@ -397,5 +439,17 @@ void BraveTabStrip::Layout() {
   TabStrip::Layout();
 }
 
+void BraveTabStrip::OnPaintBackground(gfx::Canvas* canvas) {
+  // Unlike upstream, we are painting this view to an opaque layer in order to
+  // support layer-based shadows under the active tab. Paint a background so
+  // that all pixels are painted appropriately.
+  ui::ColorId color_id = ShouldShowVerticalTabs() ? kColorToolbar
+                         : GetWidget()->ShouldPaintAsActive()
+                             ? kColorTabBackgroundInactiveFrameActive
+                             : kColorTabBackgroundInactiveFrameInactive;
+
+  canvas->DrawColor(GetColorProvider()->GetColor(color_id));
+}
+
 BEGIN_METADATA(BraveTabStrip, TabStrip)
 END_METADATA
diff --git a/browser/ui/views/tabs/brave_tab_strip.h b/browser/ui/views/tabs/brave_tab_strip.h
index 2382c7a384c4..fee0d06262ef 100644
--- a/browser/ui/views/tabs/brave_tab_strip.h
+++ b/browser/ui/views/tabs/brave_tab_strip.h
@@ -37,6 +37,7 @@ class BraveTabStrip : public TabStrip {
   FRIEND_TEST_ALL_PREFIXES(ColorPaletteTest, LightThemeMinimumContrast);
 
   void UpdateTabContainer();
+  void UpdateTabStripMargins();
   bool ShouldShowVerticalTabs() const;
 
   // TabStrip overrides:
@@ -44,6 +45,7 @@ class BraveTabStrip : public TabStrip {
   bool ShouldDrawStrokes() const override;
   int GetStrokeThickness() const override;
   void Layout() override;
+  void OnPaintBackground(gfx::Canvas* canvas) override;
 
   // Exposed for testing.
   static constexpr float kBraveMinimumContrastRatioForOutlines = 1.2797f;
diff --git a/browser/ui/views/tabs/brave_tab_strip_layout_helper.cc b/browser/ui/views/tabs/brave_tab_strip_layout_helper.cc
index 477320131e6e..fa49eb2fabd0 100644
--- a/browser/ui/views/tabs/brave_tab_strip_layout_helper.cc
+++ b/browser/ui/views/tabs/brave_tab_strip_layout_helper.cc
@@ -7,6 +7,8 @@
 
 #include <limits>
 
+#include "brave/browser/ui/tabs/brave_tab_layout_constants.h"
+#include "brave/browser/ui/tabs/features.h"
 #include "brave/browser/ui/views/tabs/brave_tab_group_header.h"
 #include "brave/browser/ui/views/tabs/brave_tab_strip.h"
 #include "chrome/browser/ui/layout_constants.h"
@@ -109,6 +111,14 @@ void CalculateVerticalLayout(const TabLayoutConstants& layout_constants,
 
 }  // namespace
 
+int GetTabCornerRadius(const Tab& tab) {
+  if (!tabs::features::HorizontalTabsUpdateEnabled()) {
+    return tab.data().pinned ? 8 : 4;
+  }
+
+  return brave_tabs::kTabBorderRadius;
+}
+
 std::vector<gfx::Rect> CalculateVerticalTabBounds(
     const TabLayoutConstants& layout_constants,
     const std::vector<TabWidthConstraints>& tabs,
@@ -127,6 +137,22 @@ std::vector<gfx::Rect> CalculateVerticalTabBounds(
   return bounds;
 }
 
+std::vector<gfx::Rect> CalculateBoundsForHorizontalDraggedViews(
+    const std::vector<TabSlotView*>& views,
+    TabStrip* tab_strip) {
+  // Chromium aligns the dragged tabs to the bottom of the tab strip, whereas we
+  // need to keep the tabs aligned to the top.
+  std::vector<gfx::Rect> bounds;
+  const int overlap = TabStyle::Get()->GetTabOverlap();
+  int x = 0;
+  for (const TabSlotView* view : views) {
+    const int width = view->width();
+    bounds.emplace_back(x, 0, width, view->height());
+    x += width - overlap;
+  }
+  return bounds;
+}
+
 std::vector<gfx::Rect> CalculateBoundsForVerticalDraggedViews(
     const std::vector<TabSlotView*>& views,
     TabStrip* tab_strip) {
diff --git a/browser/ui/views/tabs/brave_tab_strip_layout_helper.h b/browser/ui/views/tabs/brave_tab_strip_layout_helper.h
index 63e54e35ccbe..6cdf8f28dea0 100644
--- a/browser/ui/views/tabs/brave_tab_strip_layout_helper.h
+++ b/browser/ui/views/tabs/brave_tab_strip_layout_helper.h
@@ -16,6 +16,7 @@ namespace gfx {
 class Rect;
 }  // namespace gfx
 
+class Tab;
 class TabStripLayoutHelper;
 class TabWidthConstraints;
 class TabStripController;
@@ -31,8 +32,7 @@ constexpr int kVerticalTabMinWidth = kVerticalTabHeight;
 constexpr int kVerticalTabsSpacing = 4;
 constexpr int kMarginForVerticalTabContainers = kVerticalTabsSpacing;
 
-constexpr int kUnpinnedTabBorderRadius = 4;
-constexpr int kPinnedTabBorderRadius = 8;
+int GetTabCornerRadius(const Tab& tab);
 
 std::vector<gfx::Rect> CalculateVerticalTabBounds(
     const TabLayoutConstants& layout_constants,
@@ -40,6 +40,10 @@ std::vector<gfx::Rect> CalculateVerticalTabBounds(
     absl::optional<int> width,
     bool is_floating_mode);
 
+std::vector<gfx::Rect> CalculateBoundsForHorizontalDraggedViews(
+    const std::vector<TabSlotView*>& views,
+    TabStrip* tab_strip);
+
 std::vector<gfx::Rect> CalculateBoundsForVerticalDraggedViews(
     const std::vector<TabSlotView*>& views,
     TabStrip* tab_strip);
diff --git a/browser/ui/views/tabs/brave_tab_style_views.inc.cc b/browser/ui/views/tabs/brave_tab_style_views.inc.cc
index 92de8fe5ddbb..fb65bbaee4f1 100644
--- a/browser/ui/views/tabs/brave_tab_style_views.inc.cc
+++ b/browser/ui/views/tabs/brave_tab_style_views.inc.cc
@@ -8,6 +8,8 @@
 
 namespace {
 
+using tabs::features::HorizontalTabsUpdateEnabled;
+
 ////////////////////////////////////////////////////////////////////////////////
 // BraveGM2TabStyle
 //
@@ -21,9 +23,6 @@ class BraveGM2TabStyle : public GM2TabStyleViews {
  protected:
   TabStyle::TabColors CalculateTargetColors() const override;
 
-  Tab* tab() { return base::to_address(tab_); }
-  const Tab* tab() const { return base::to_address(tab_); }
-
  private:
   raw_ptr<Tab> tab_;
 };
@@ -63,12 +62,14 @@ class BraveVerticalTabStyle : public BraveGM2TabStyle {
                  bool force_active = false,
                  TabStyle::RenderUnits render_units =
                      TabStyle::RenderUnits::kPixels) const override;
+
+  gfx::Insets GetContentsInsets() const override;
   TabStyle::SeparatorBounds GetSeparatorBounds(float scale) const override;
+  float GetSeparatorOpacity(bool for_layout, bool leading) const override;
   void PaintTab(gfx::Canvas* canvas) const override;
 
  private:
   bool ShouldShowVerticalTabs() const;
-  bool IsInGroupAndNotActive() const;
   SkColor GetTargetTabBackgroundColor(
       TabStyle::TabSelectionState selection_state,
       bool hovered) const override;
@@ -82,7 +83,7 @@ SkPath BraveVerticalTabStyle::GetPath(
     float scale,
     bool force_active,
     TabStyle::RenderUnits render_units) const {
-  if (!ShouldShowVerticalTabs()) {
+  if (!HorizontalTabsUpdateEnabled() && !ShouldShowVerticalTabs()) {
     return BraveGM2TabStyle::GetPath(path_type, scale, force_active,
                                      render_units);
   }
@@ -94,6 +95,15 @@ SkPath BraveVerticalTabStyle::GetPath(
     return {};
   }
 
+  // Horizontal tabs should have a visual gap between them, even if their view
+  // bounds are touching or slightly overlapping. Create a visual gap by
+  // insetting the bounds of the tab by the required gap plus overlap before
+  // drawing the rectangle.
+  if (!ShouldShowVerticalTabs()) {
+    aligned_bounds.Inset(
+        gfx::InsetsF::VH(0, brave_tabs::kHorizontalTabInset * scale));
+  }
+
 #if DCHECK_IS_ON()
   if (tab()->bounds().height() != std::round(aligned_bounds.height() / scale)) {
     DLOG(ERROR) << "We don't want it to be off by 1 dip\n|height|: "
@@ -109,8 +119,7 @@ SkPath BraveVerticalTabStyle::GetPath(
   float tab_left = aligned_bounds.x();
   float tab_right = aligned_bounds.right();
   float tab_bottom = aligned_bounds.bottom();
-  int radius =
-      is_pinned ? tabs::kPinnedTabBorderRadius : tabs::kUnpinnedTabBorderRadius;
+  int radius = tabs::GetTabCornerRadius(*tab());
 
   if (is_pinned) {
     // Only pinned tabs have border
@@ -152,28 +161,111 @@ SkPath BraveVerticalTabStyle::GetPath(
   return path;
 }
 
+gfx::Insets BraveVerticalTabStyle::GetContentsInsets() const {
+  if (!HorizontalTabsUpdateEnabled()) {
+    return BraveGM2TabStyle::GetContentsInsets();
+  }
+
+  // Ignore any stroke widths when determining the horizontal contents insets.
+  return tab_style()->GetContentsInsets();
+}
+
 TabStyle::SeparatorBounds BraveVerticalTabStyle::GetSeparatorBounds(
     float scale) const {
   if (ShouldShowVerticalTabs()) {
     return {};
   }
 
-  return BraveGM2TabStyle::GetSeparatorBounds(scale);
+  if (!HorizontalTabsUpdateEnabled()) {
+    return BraveGM2TabStyle::GetSeparatorBounds(scale);
+  }
+
+  gfx::SizeF size(tab_style()->GetSeparatorSize());
+  size.Scale(scale);
+  const gfx::RectF aligned_bounds =
+      ScaleAndAlignBounds(tab()->bounds(), scale, GetStrokeThickness());
+
+  // Note: `leading` bounds are used for rect corner radius calculation and so
+  // must be non-empty, even if we don't want to show it.
+  TabStyle::SeparatorBounds bounds;
+  bounds.leading = gfx::RectF(aligned_bounds.right(),
+                              (aligned_bounds.height() - size.height()) / 2,
+                              size.width(), size.height());
+  bounds.trailing = bounds.leading;
+  bounds.trailing.set_x(aligned_bounds.right() - size.width());
+
+  gfx::PointF origin(tab()->bounds().origin());
+  origin.Scale(scale);
+  bounds.trailing.Offset(-origin.x(), -origin.y());
+
+  return bounds;
+}
+
+float BraveVerticalTabStyle::GetSeparatorOpacity(bool for_layout,
+                                                 bool leading) const {
+  if (ShouldShowVerticalTabs() || !HorizontalTabsUpdateEnabled()) {
+    return BraveGM2TabStyle::GetSeparatorOpacity(for_layout, leading);
+  }
+
+  if (leading) {
+    return 0;
+  }
+
+  if (tab()->data().pinned) {
+    return 0;
+  }
+
+  const auto has_visible_background = [](const Tab* const tab) {
+    return tab->IsActive() || tab->IsSelected() || tab->IsMouseHovered();
+  };
+
+  if (has_visible_background(tab())) {
+    return 0;
+  }
+
+  const Tab* const next_tab = tab()->controller()->GetAdjacentTab(tab(), 1);
+
+  const float visible_opacity =
+      GetHoverInterpolatedSeparatorOpacity(for_layout, next_tab);
+
+  // Show separator if this is the last tab (and is therefore followed by the
+  // new tab icon).
+  if (!next_tab) {
+    return visible_opacity;
+  }
+
+  // Show separator if there is a group header between this tab and the next.
+  if (next_tab->group().has_value() && tab()->group() != next_tab->group()) {
+    return visible_opacity;
+  }
+
+  if (has_visible_background(next_tab)) {
+    return 0;
+  }
+
+  return visible_opacity;
 }
 
 void BraveVerticalTabStyle::PaintTab(gfx::Canvas* canvas) const {
   BraveGM2TabStyle::PaintTab(canvas);
-  if (!ShouldShowVerticalTabs()) {
+
+  if (!HorizontalTabsUpdateEnabled() && !ShouldShowVerticalTabs()) {
     return;
   }
 
   if (tab()->data().pinned) {
     const auto* widget = tab()->GetWidget();
     CHECK(widget);
-    const SkColor tab_stroke_color =
-        widget->GetColorProvider()->GetColor(kColorBraveVerticalTabSeparator);
+
+    ui::ColorId color_id = widget->ShouldPaintAsActive()
+                               ? kColorTabStrokeFrameActive
+                               : kColorTabStrokeFrameInactive;
+    if (ShouldShowVerticalTabs()) {
+      color_id = kColorBraveVerticalTabSeparator;
+    }
+
     PaintBackgroundStroke(canvas, TabStyle::TabSelectionState::kActive,
-                          tab_stroke_color);
+                          widget->GetColorProvider()->GetColor(color_id));
   }
 }
 
diff --git a/chromium_src/chrome/browser/ui/tabs/tab_style.cc b/chromium_src/chrome/browser/ui/tabs/tab_style.cc
new file mode 100644
index 000000000000..98fe3ddb3e51
--- /dev/null
+++ b/chromium_src/chrome/browser/ui/tabs/tab_style.cc
@@ -0,0 +1,10 @@
+/* Copyright (c) 2023 The Brave Authors. All rights reserved.
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+#include "brave/browser/ui/tabs/brave_tab_style.h"
+
+#define BRAVE_TAB_STYLE_GET return new BraveTabStyle<GM2TabStyle>();
+#include "src/chrome/browser/ui/tabs/tab_style.cc"
+#undef BRAVE_TAB_STYLE_GET
diff --git a/chromium_src/chrome/browser/ui/views/frame/browser_frame_view_win.cc b/chromium_src/chrome/browser/ui/views/frame/browser_frame_view_win.cc
new file mode 100644
index 000000000000..c909aca6abde
--- /dev/null
+++ b/chromium_src/chrome/browser/ui/views/frame/browser_frame_view_win.cc
@@ -0,0 +1,17 @@
+/* Copyright (c) 2023 The Brave Authors. All rights reserved.
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+#include "brave/browser/ui/tabs/features.h"
+
+// The tab region view maintains its own padding; the frame view does not need
+// to reserve an extra top margin for it.
+#define BRAVE_BROWSER_FRAME_VIEW_WIN_TOP_AREA_HEIGHT   \
+  if (tabs::features::HorizontalTabsUpdateEnabled()) { \
+    return top;                                        \
+  }
+
+#include "src/chrome/browser/ui/views/frame/browser_frame_view_win.cc"  // IWYU pragma: export
+
+#undef BRAVE_BROWSER_FRAME_VIEW_WIN_TOP_AREA_HEIGHT
diff --git a/chromium_src/chrome/browser/ui/views/frame/tab_strip_region_view.cc b/chromium_src/chrome/browser/ui/views/frame/tab_strip_region_view.cc
index 8eb14116f21c..844e43a854a7 100644
--- a/chromium_src/chrome/browser/ui/views/frame/tab_strip_region_view.cc
+++ b/chromium_src/chrome/browser/ui/views/frame/tab_strip_region_view.cc
@@ -6,8 +6,17 @@
 #include "brave/browser/ui/views/tabs/brave_new_tab_button.h"
 #include "brave/browser/ui/views/tabs/brave_tab_search_button.h"
 
+// Currently, `NewTabButton::kButtonSize` is used to calculate tab strip button
+// borders. Since the size of buttons varies depending upon the tabs update
+// feature flag, replace the `kButtonSize` identifier with a call to the static
+// `BraveNewTabButton::GetButtonSize` function.
+#define kButtonSize GetButtonSize()
+
 #define NewTabButton BraveNewTabButton
 #define TabSearchButton BraveTabSearchButton
+
 #include "src/chrome/browser/ui/views/frame/tab_strip_region_view.cc"
+
 #undef TabSearchButton
 #undef NewTabButton
+#undef kButtonSize
diff --git a/chromium_src/chrome/browser/ui/views/tabs/tab_group_style.cc b/chromium_src/chrome/browser/ui/views/tabs/tab_group_style.cc
index 8f1eee8aac46..bd78646c6f6e 100644
--- a/chromium_src/chrome/browser/ui/views/tabs/tab_group_style.cc
+++ b/chromium_src/chrome/browser/ui/views/tabs/tab_group_style.cc
@@ -5,6 +5,8 @@
 
 #include "chrome/browser/ui/views/tabs/tab_group_style.h"
 
+#include "brave/browser/ui/tabs/brave_tab_layout_constants.h"
+#include "brave/browser/ui/tabs/features.h"
 #include "brave/browser/ui/views/tabs/vertical_tab_utils.h"
 
 #define TabGroupStyle TabGroupStyle_ChromiumImpl
@@ -48,10 +50,28 @@ SkPath TabGroupStyle::GetUnderlinePath(gfx::Rect local_bounds) const {
   return path;
 }
 
+gfx::Insets TabGroupStyle::GetInsetsForHeaderChip() const {
+  auto insets = TabGroupStyle_ChromiumImpl::GetInsetsForHeaderChip();
+  if (!tabs::features::HorizontalTabsUpdateEnabled()) {
+    return insets;
+  }
+  if (!ShouldShowVerticalTabs()) {
+    insets += gfx::Insets::VH(0, brave_tabs::kHorizontalTabInset);
+  }
+  return insets;
+}
+
 bool TabGroupStyle::ShouldShowVerticalTabs() const {
   return tabs::utils::ShouldShowVerticalTabs(tab_group_views_->GetBrowser());
 }
 
+float TabGroupStyle::GetEmptyChipSize() const {
+  if (!tabs::features::HorizontalTabsUpdateEnabled()) {
+    return TabGroupStyle_ChromiumImpl::GetEmptyChipSize();
+  }
+  return brave_tabs::kEmptyGroupTitleSize;
+}
+
 int ChromeRefresh2023TabGroupStyle::GetTabGroupOverlapAdjustment() {
   return ChromeRefresh2023TabGroupStyle_ChromiumImpl::
       GetTabGroupOverlapAdjustment();
diff --git a/chromium_src/chrome/browser/ui/views/tabs/tab_group_style.h b/chromium_src/chrome/browser/ui/views/tabs/tab_group_style.h
index 7b35fcf3f323..432645d836be 100644
--- a/chromium_src/chrome/browser/ui/views/tabs/tab_group_style.h
+++ b/chromium_src/chrome/browser/ui/views/tabs/tab_group_style.h
@@ -22,6 +22,10 @@ class TabGroupStyle : public TabGroupStyle_ChromiumImpl {
 
   SkPath GetUnderlinePath(gfx::Rect local_bounds) const override;
 
+  gfx::Insets GetInsetsForHeaderChip() const override;
+
+  float GetEmptyChipSize() const override;
+
  private:
   bool ShouldShowVerticalTabs() const;
 };
diff --git a/chromium_src/chrome/browser/ui/views/tabs/tab_strip.cc b/chromium_src/chrome/browser/ui/views/tabs/tab_strip.cc
index 5bafabdef260..49bf083fc432 100644
--- a/chromium_src/chrome/browser/ui/views/tabs/tab_strip.cc
+++ b/chromium_src/chrome/browser/ui/views/tabs/tab_strip.cc
@@ -7,6 +7,7 @@
 
 #include <cmath>
 
+#include "brave/browser/ui/tabs/features.h"
 #include "brave/browser/ui/views/tabs/brave_compound_tab_container.h"
 #include "brave/browser/ui/views/tabs/brave_tab.h"
 #include "brave/browser/ui/views/tabs/brave_tab_group_header.h"
@@ -43,19 +44,20 @@
     continue;                                                                \
   }
 
-#define BRAVE_TAB_DRAG_CONTEXT_IMPL_CALCULATE_BOUNDS_FOR_DRAGGED_VIEWS      \
-  if (tabs::utils::ShouldShowVerticalTabs(tab_strip_->GetBrowser())) {      \
-    return tabs::CalculateBoundsForVerticalDraggedViews(views, tab_strip_); \
+#define BRAVE_TAB_DRAG_CONTEXT_IMPL_CALCULATE_BOUNDS_FOR_DRAGGED_VIEWS        \
+  if (tabs::utils::ShouldShowVerticalTabs(tab_strip_->GetBrowser())) {        \
+    return tabs::CalculateBoundsForVerticalDraggedViews(views, tab_strip_);   \
+  }                                                                           \
+  if (tabs::features::HorizontalTabsUpdateEnabled()) {                        \
+    return tabs::CalculateBoundsForHorizontalDraggedViews(views, tab_strip_); \
   }
 
-#define BRAVE_TAB_DRAG_CONTEXT_IMPL_PAINT_CHILDREN                        \
-  if (tabs::utils::ShouldShowVerticalTabs(tab_strip_->GetBrowser())) {    \
-    for (const ZOrderableTabContainerElement& child : orderable_children) \
-      if (!child.view()->layer()) {                                       \
-        child.view()->Paint(paint_info);                                  \
-      }                                                                   \
-    return;                                                               \
-  }
+#define BRAVE_TAB_DRAG_CONTEXT_IMPL_PAINT_CHILDREN                      \
+  for (const ZOrderableTabContainerElement& child : orderable_children) \
+    if (!child.view()->layer()) {                                       \
+      child.view()->Paint(paint_info);                                  \
+    }                                                                   \
+  return;
 
 #include "src/chrome/browser/ui/views/tabs/tab_strip.cc"
 
diff --git a/chromium_src/chrome/browser/ui/views/tabs/tab_style_views.cc b/chromium_src/chrome/browser/ui/views/tabs/tab_style_views.cc
index 15a78ec814eb..fbd4bff6e420 100644
--- a/chromium_src/chrome/browser/ui/views/tabs/tab_style_views.cc
+++ b/chromium_src/chrome/browser/ui/views/tabs/tab_style_views.cc
@@ -5,6 +5,8 @@
 
 #include "chrome/browser/ui/views/tabs/tab_style_views.h"
 #include "brave/browser/ui/color/brave_color_id.h"
+#include "brave/browser/ui/tabs/brave_tab_layout_constants.h"
+#include "brave/browser/ui/tabs/features.h"
 #include "brave/browser/ui/views/tabs/brave_tab_group_header.h"
 #include "brave/browser/ui/views/tabs/vertical_tab_utils.h"
 #include "chrome/browser/ui/views/tabs/tab_slot_controller.h"
diff --git a/patches/chrome-browser-ui-tabs-tab_style.cc.patch b/patches/chrome-browser-ui-tabs-tab_style.cc.patch
new file mode 100644
index 000000000000..ebf2e43fcbd7
--- /dev/null
+++ b/patches/chrome-browser-ui-tabs-tab_style.cc.patch
@@ -0,0 +1,12 @@
+diff --git a/chrome/browser/ui/tabs/tab_style.cc b/chrome/browser/ui/tabs/tab_style.cc
+index aa9cd303743116892d65f71f66759bc4cefe05f5..25a252b713af07ecc4cf3ecd0312d48b9f9eba3a 100644
+--- a/chrome/browser/ui/tabs/tab_style.cc
++++ b/chrome/browser/ui/tabs/tab_style.cc
+@@ -275,6 +275,7 @@ SkColor ChromeRefresh2023TabStyle::GetTabBackgroundColor(
+ 
+ // static
+ const TabStyle* TabStyle::Get() {
++  BRAVE_TAB_STYLE_GET
+   static TabStyle* const tab_style =
+       features::IsChromeRefresh2023()
+           ? static_cast<TabStyle*>(new ChromeRefresh2023TabStyle())
diff --git a/patches/chrome-browser-ui-views-frame-browser_frame_view_win.cc.patch b/patches/chrome-browser-ui-views-frame-browser_frame_view_win.cc.patch
new file mode 100644
index 000000000000..0816ed9c34a7
--- /dev/null
+++ b/patches/chrome-browser-ui-views-frame-browser_frame_view_win.cc.patch
@@ -0,0 +1,12 @@
+diff --git a/chrome/browser/ui/views/frame/browser_frame_view_win.cc b/chrome/browser/ui/views/frame/browser_frame_view_win.cc
+index a4c7824a16a504f577668cfb2b40428c1b63d1c1..bde9e6ede746e52cd90c7417917b503c635eb79e 100644
+--- a/chrome/browser/ui/views/frame/browser_frame_view_win.cc
++++ b/chrome/browser/ui/views/frame/browser_frame_view_win.cc
+@@ -531,6 +531,7 @@ int BrowserFrameViewWin::TopAreaHeight(bool restored) const {
+                      : caption_button_container_->GetPreferredSize().height();
+     return top;
+   }
++  BRAVE_BROWSER_FRAME_VIEW_WIN_TOP_AREA_HEIGHT
+ 
+   // In Refresh, the tabstrip controls its own top padding.
+   if (features::IsChromeRefresh2023()) {