diff --git a/lib/active_model/serializer/association.rb b/lib/active_model/serializer/association.rb
index 7aeee338a..395573c12 100644
--- a/lib/active_model/serializer/association.rb
+++ b/lib/active_model/serializer/association.rb
@@ -58,7 +58,7 @@ def serializable_hash(adapter_options, adapter_instance)
         serialization = association_serializer.serializable_hash(adapter_options, {}, adapter_instance)
 
         if polymorphic? && serialization
-          polymorphic_type = association_object.class.name.underscore
+          polymorphic_type = association_serializer.json_key || association_object.class.name.underscore
           serialization = { type: polymorphic_type, polymorphic_type.to_sym => serialization }
         end
 
diff --git a/test/adapter/polymorphic_test.rb b/test/adapter/polymorphic_test.rb
index 48bcd979e..558e5165f 100644
--- a/test/adapter/polymorphic_test.rb
+++ b/test/adapter/polymorphic_test.rb
@@ -12,8 +12,8 @@ class PolymorphicTest < ActiveSupport::TestCase
           @picture.imageable = @employee
         end
 
-        def serialization(resource, adapter = :attributes)
-          serializable(resource, adapter: adapter, serializer: PolymorphicBelongsToSerializer).as_json
+        def serialization(resource, adapter: :attributes, serializer: PolymorphicBelongsToSerializer)
+          serializable(resource, adapter: adapter, serializer: serializer).as_json
         end
 
         def tag_serialization(adapter = :attributes)
@@ -40,6 +40,23 @@ def test_attributes_serialization
           assert_equal(expected, serialization(@picture))
         end
 
+        def test_polymorphic_belongs_to_with_custom_type
+          expected =
+          {
+            id: 1,
+            title: 'headshot-1.jpg',
+            imageable: {
+              type: 'custom',
+              custom: {
+                id: 42,
+                name: 'Zoop Zoopler'
+              }
+            }
+          }
+
+          assert_equal(expected, serialization(@picture, serializer: PolymorphicBelongsToSerializerWithCustomBelongsToType))
+        end
+
         def test_attributes_serialization_without_polymorphic_association
           expected =
             {
@@ -97,7 +114,7 @@ def test_json_serialization
               }
             }
 
-          assert_equal(expected, serialization(@picture, :json))
+          assert_equal(expected, serialization(@picture, adapter: :json))
         end
 
         def test_json_serialization_without_polymorphic_association
@@ -111,7 +128,7 @@ def test_json_serialization_without_polymorphic_association
             }
 
           simple_picture = Picture.new(id: 2, title: 'headshot-2.jpg')
-          assert_equal(expected, serialization(simple_picture, :json))
+          assert_equal(expected, serialization(simple_picture, adapter: :json))
         end
 
         def test_json_serialization_with_polymorphic_has_many
@@ -165,7 +182,7 @@ def test_json_api_serialization
               }
             }
 
-          assert_equal(expected, serialization(@picture, :json_api))
+          assert_equal(expected, serialization(@picture, adapter: :json_api))
         end
 
         def test_json_api_serialization_with_polymorphic_belongs_to
diff --git a/test/fixtures/active_record.rb b/test/fixtures/active_record.rb
index 132a43b16..9afd1b5ca 100644
--- a/test/fixtures/active_record.rb
+++ b/test/fixtures/active_record.rb
@@ -113,3 +113,13 @@ class PolymorphicBelongsToSerializer < ActiveModel::Serializer
   attributes :id, :title
   belongs_to :imageable, serializer: PolymorphicHasManySerializer, polymorphic: true
 end
+
+class PolymorphicHasManySerializerWithCustomType < ActiveModel::Serializer
+  attributes :id, :name
+
+  type :custom
+end
+class PolymorphicBelongsToSerializerWithCustomBelongsToType < ActiveModel::Serializer
+  attributes :id, :title
+  belongs_to :imageable, serializer: PolymorphicHasManySerializerWithCustomType, polymorphic: true
+end