diff --git a/Gemfile b/Gemfile
index a808f9a3872..d9c4c11dcdd 100644
--- a/Gemfile
+++ b/Gemfile
@@ -106,6 +106,7 @@ gem 'maxmind-db', '~> 1.0.0'
 gem 'mahoro', '~> 0.5'
 gem 'nokogiri', '~> 1.14.1'
 gem 'open4', '~> 1.3.0'
+gem 'puma', '~> 6.0.2'
 gem 'rack', '~> 2.2.6'
 gem 'rack-utf8_sanitizer', '~> 1.8.0'
 gem 'recaptcha', '~> 5.12.3', require: 'recaptcha/rails'
@@ -120,7 +121,6 @@ gem 'strip_attributes', :git => 'https://github.com/mysociety/strip_attributes.g
 gem 'stripe', '~> 5.55.0'
 gem 'syck', '~> 1.4.1', require: false
 gem 'syslog_protocol', '~> 0.9.0'
-gem 'thin', '~> 1.8.1'
 gem 'vpim', '~> 13.11.11'
 gem 'will_paginate', '~> 3.3.1'
 gem 'xapian-full-alaveteli', '~> 1.4.21.1'
diff --git a/Gemfile.lock b/Gemfile.lock
index 97b2a05c40e..fbcf9f5900b 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -174,7 +174,6 @@ GEM
     crack (0.4.5)
       rexml
     crass (1.0.6)
-    daemons (1.4.0)
     dalli (3.2.3)
     dante (0.2.0)
     date (3.3.3)
@@ -184,7 +183,6 @@ GEM
       rake (>= 12.0.0, < 14.0.0)
     docile (1.4.0)
     erubi (1.12.0)
-    eventmachine (1.2.7)
     exception_notification (4.5.0)
       actionmailer (>= 5.2, < 8)
       activesupport (>= 5.2, < 8)
@@ -358,6 +356,8 @@ GEM
       coderay (~> 1.1)
       method_source (~> 1.0)
     public_suffix (5.0.1)
+    puma (6.0.2)
+      nio4r (~> 2.0)
     racc (1.6.2)
     rack (2.2.6.2)
     rack-test (2.0.2)
@@ -494,10 +494,6 @@ GEM
     syck (1.4.1)
     syslog_protocol (0.9.2)
     text (1.3.1)
-    thin (1.8.1)
-      daemons (~> 1.0, >= 1.0.9)
-      eventmachine (~> 1.0, >= 1.0.4)
-      rack (>= 1, < 3)
     thor (1.2.1)
     tilt (2.0.10)
     timeout (0.3.1)
@@ -593,6 +589,7 @@ DEPENDENCIES
   open4 (~> 1.3.0)
   pg (~> 1.4.5)
   pry (~> 0.14.2)
+  puma (~> 6.0.2)
   rack (~> 2.2.6)
   rack-utf8_sanitizer (~> 1.8.0)
   rails (~> 7.0.4)
@@ -618,7 +615,6 @@ DEPENDENCIES
   stripe-ruby-mock!
   syck (~> 1.4.1)
   syslog_protocol (~> 0.9.0)
-  thin (~> 1.8.1)
   uglifier (~> 4.2.0)
   unicode (~> 0.4.4)
   unidecoder (~> 1.1.0)
diff --git a/bin/puma b/bin/puma
new file mode 100755
index 00000000000..01a92a32422
--- /dev/null
+++ b/bin/puma
@@ -0,0 +1,27 @@
+#!/usr/bin/env ruby
+# frozen_string_literal: true
+
+#
+# This file was generated by Bundler.
+#
+# The application 'puma' is installed as part of a gem, and
+# this file is here to facilitate running it.
+#
+
+ENV["BUNDLE_GEMFILE"] ||= File.expand_path("../Gemfile", __dir__)
+
+bundle_binstub = File.expand_path("bundle", __dir__)
+
+if File.file?(bundle_binstub)
+  if File.read(bundle_binstub, 300).include?("This file was generated by Bundler")
+    load(bundle_binstub)
+  else
+    abort("Your `bin/bundle` was not generated by Bundler, so this binstub cannot run.
+Replace `bin/bundle` by running `bundle binstubs bundler --force`, then run this command again.")
+  end
+end
+
+require "rubygems"
+require "bundler/setup"
+
+load Gem.bin_path("puma", "puma")
diff --git a/bin/pumactl b/bin/pumactl
new file mode 100755
index 00000000000..c93cff2107e
--- /dev/null
+++ b/bin/pumactl
@@ -0,0 +1,27 @@
+#!/usr/bin/env ruby
+# frozen_string_literal: true
+
+#
+# This file was generated by Bundler.
+#
+# The application 'pumactl' is installed as part of a gem, and
+# this file is here to facilitate running it.
+#
+
+ENV["BUNDLE_GEMFILE"] ||= File.expand_path("../Gemfile", __dir__)
+
+bundle_binstub = File.expand_path("bundle", __dir__)
+
+if File.file?(bundle_binstub)
+  if File.read(bundle_binstub, 300).include?("This file was generated by Bundler")
+    load(bundle_binstub)
+  else
+    abort("Your `bin/bundle` was not generated by Bundler, so this binstub cannot run.
+Replace `bin/bundle` by running `bundle binstubs bundler --force`, then run this command again.")
+  end
+end
+
+require "rubygems"
+require "bundler/setup"
+
+load Gem.bin_path("puma", "pumactl")
diff --git a/bin/thin b/bin/thin
deleted file mode 100755
index 80d4e141010..00000000000
--- a/bin/thin
+++ /dev/null
@@ -1,16 +0,0 @@
-#!/usr/bin/env ruby
-#
-# This file was generated by Bundler.
-#
-# The application 'thin' is installed as part of a gem, and
-# this file is here to facilitate running it.
-#
-
-require 'pathname'
-ENV['BUNDLE_GEMFILE'] ||= File.expand_path("../../Gemfile",
-  Pathname.new(__FILE__).realpath)
-
-require 'rubygems'
-require 'bundler/setup'
-
-load Gem.bin_path('thin', 'thin')
diff --git a/config/boot.rb b/config/boot.rb
index e6a5f1c48da..5df8ee545d5 100644
--- a/config/boot.rb
+++ b/config/boot.rb
@@ -1,10 +1,4 @@
 ENV['BUNDLE_GEMFILE'] ||= File.expand_path('../Gemfile', __dir__)
 
 require "bundler/setup" # Set up gems listed in the Gemfile.
-
-# TODO: Remove this. This is a hacky system for having a default environment.
-# It looks for a config/rails_env.rb file, and reads stuff from there if
-# it exists. Put just a line like this in there:
-#   ENV['RAILS_ENV'] = 'production'
-rails_env_file = File.expand_path(File.join(File.dirname(__FILE__), 'rails_env.rb'))
-require rails_env_file if File.exist?(rails_env_file)
+require File.expand_path('load_env.rb', __dir__)
diff --git a/config/load_env.rb b/config/load_env.rb
new file mode 100644
index 00000000000..3b257efd3af
--- /dev/null
+++ b/config/load_env.rb
@@ -0,0 +1,7 @@
+# TODO: Remove this. This is a hacky system for having a default environment.
+# It looks for a config/rails_env.rb file, and reads stuff from there if
+# it exists. Put just a line like this in there:
+#   ENV['RAILS_ENV'] = 'production'
+
+rails_env_file = File.expand_path('rails_env.rb', __dir__)
+require rails_env_file if File.exist?(rails_env_file)
diff --git a/config/puma.rb b/config/puma.rb
new file mode 100644
index 00000000000..452301a592a
--- /dev/null
+++ b/config/puma.rb
@@ -0,0 +1,41 @@
+require File.expand_path('load_env.rb', __dir__)
+
+# This configuration file will be evaluated by Puma. The top-level methods that
+# are invoked here are part of Puma's configuration DSL. For more information
+# about methods provided by the DSL, see https://puma.io/puma/Puma/DSL.html.
+
+# Puma can serve each request in a thread from an internal thread pool.
+# The `threads` method setting takes two numbers: a minimum and maximum.
+# Any libraries that use thread pools should be configured to match
+# the maximum value specified for Puma. Default is set to 5 threads for minimum
+# and maximum; this matches the default thread size of Active Record.
+max_threads_count = ENV.fetch("RAILS_MAX_THREADS") { 5 }
+min_threads_count = ENV.fetch("RAILS_MIN_THREADS") { max_threads_count }
+threads min_threads_count, max_threads_count
+
+# Specifies that the worker count should equal the number of processors in
+# production.
+if ENV["RAILS_ENV"] == "production"
+  concurrency = ENV.fetch("WEB_CONCURRENCY") do
+    Concurrent.physical_processor_count
+  end
+  worker_count = Integer(concurrency)
+  workers worker_count if worker_count > 1
+end
+
+# Specifies the `worker_timeout` threshold that Puma will use to wait before
+# terminating a worker in development environments.
+worker_timeout 3600 if ENV.fetch("RAILS_ENV", "development") == "development"
+
+# Specifies the `port` that Puma will listen on to receive requests; default is
+# 3000.
+port ENV.fetch("PORT") { 3000 }
+
+# Specifies the `environment` that Puma will run in.
+environment ENV.fetch("RAILS_ENV") { "development" }
+
+# Specifies the `pidfile` that Puma will use.
+pidfile ENV.fetch("PIDFILE") { "tmp/pids/server.pid" }
+
+# Allow puma to be restarted by `bin/rails restart` command.
+plugin :tmp_restart
diff --git a/config/sysvinit-thin.example b/config/sysvinit-puma.example
similarity index 95%
rename from config/sysvinit-thin.example
rename to config/sysvinit-puma.example
index 987f3ac233a..b3539598500 100755
--- a/config/sysvinit-thin.example
+++ b/config/sysvinit-puma.example
@@ -1,7 +1,7 @@
 #! /bin/sh
 #
 ### BEGIN INIT INFO
-# Provides:          application-thin-<%= site %>
+# Provides:          application-puma-<%= site %>
 # Required-Start:    $local_fs $network
 # Required-Stop:     $local_fs $network
 # Default-Start:     2 3 4 5
@@ -23,7 +23,7 @@ CPUS=<%= cpus %>
 # RAILS_ENV=<%= rails_env %>
 # export RAILS_ENV
 
-CMD="bundle exec thin"
+CMD="bundle exec puma -C config/puma.rb"
 
 <% if use_rbenv? %>
 RBENV_ROOT="/home/<%= user %>/.rbenv"
diff --git a/script/site-specific-install.sh b/script/site-specific-install.sh
index 9a456170900..9c0a49e793d 100755
--- a/script/site-specific-install.sh
+++ b/script/site-specific-install.sh
@@ -287,7 +287,7 @@ echo $DONE_MSG
 
 if [ ! "$DEVELOPMENT_INSTALL" = true ]; then
   echo -n "Creating /etc/init.d/$SITE... "
-  (su -l -c "cd '$REPOSITORY' && bundle exec rake config_files:convert_init_script DEPLOY_USER='$UNIX_USER' VHOST_DIR='$DIRECTORY' VCSPATH='$SITE' SITE='$SITE' RUBY_VERSION='$RUBY_VERSION' USE_RBENV=$USE_RBENV RAILS_ENV='$RAILS_ENV' SCRIPT_FILE=config/sysvinit-thin.example" "$UNIX_USER") > /etc/init.d/"$SITE"
+  (su -l -c "cd '$REPOSITORY' && bundle exec rake config_files:convert_init_script DEPLOY_USER='$UNIX_USER' VHOST_DIR='$DIRECTORY' VCSPATH='$SITE' SITE='$SITE' RUBY_VERSION='$RUBY_VERSION' USE_RBENV=$USE_RBENV RAILS_ENV='$RAILS_ENV' SCRIPT_FILE=config/sysvinit-puma.example" "$UNIX_USER") > /etc/init.d/"$SITE"
   chgrp "$UNIX_USER" /etc/init.d/"$SITE"
   chmod 754 /etc/init.d/"$SITE"
   echo $DONE_MSG