diff --git a/CHANGELOG.md b/CHANGELOG.md
index c0ed98b7..9313e61f 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,7 @@ Breaking changes:
 Features:
 
 - [#2021](https://github.com/rails-api/active_model_serializers/pull/2021) ActiveModelSerializers::Model#attributes. Originally in [#1982](https://github.com/rails-api/active_model_serializers/pull/1982). (@bf4)
+- [#2130](https://github.com/rails-api/active_model_serializers/pull/2130) Allow serialized ID to be overwritten for belongs-to relationships. (@greysteil)
 
 Fixes:
 
diff --git a/lib/active_model_serializers/adapter/json_api/relationship.rb b/lib/active_model_serializers/adapter/json_api/relationship.rb
index 5d7399a3..45cddb89 100644
--- a/lib/active_model_serializers/adapter/json_api/relationship.rb
+++ b/lib/active_model_serializers/adapter/json_api/relationship.rb
@@ -45,7 +45,7 @@ module ActiveModelSerializers
         def data_for_one(association)
           if association.belongs_to? &&
               parent_serializer.object.respond_to?(association.reflection.foreign_key)
-            id = parent_serializer.object.send(association.reflection.foreign_key)
+            id = parent_serializer.read_attribute_for_serialization(association.reflection.foreign_key)
             type = association.reflection.type.to_s
             ResourceIdentifier.for_type_with_id(type, id, serializable_resource_options)
           else
diff --git a/test/serializers/associations_test.rb b/test/serializers/associations_test.rb
index c1b164b8..5d195cbc 100644
--- a/test/serializers/associations_test.rb
+++ b/test/serializers/associations_test.rb
@@ -165,6 +165,36 @@ module ActiveModel
         assert_equal expected, actual
       end
 
+      class ExternalBlog < Blog
+        attributes :external_id
+      end
+      class BelongsToExternalBlogModel < ::Model
+        attributes :id, :title, :external_blog_id
+        associations :external_blog
+      end
+      class BelongsToExternalBlogModelSerializer < ActiveModel::Serializer
+        type :posts
+        belongs_to :external_blog
+
+        def external_blog_id
+          object.external_blog.external_id
+        end
+      end
+
+      def test_belongs_to_allows_id_overwriting
+        attributes = {
+          id: 1,
+          title: 'Title',
+          external_blog: ExternalBlog.new(id: 5, external_id: 6)
+        }
+        post = BelongsToExternalBlogModel.new(attributes)
+
+        actual = serializable(post, adapter: :json_api, serializer: BelongsToExternalBlogModelSerializer).as_json
+        expected = { data: { id: '1', type: 'posts', relationships: { :'external-blog' => { data: { id: '6', type: 'external-blogs' } } } } }
+
+        assert_equal expected, actual
+      end
+
       class InlineAssociationTestPostSerializer < ActiveModel::Serializer
         has_many :comments
         has_many :comments, key: :last_comments do