diff --git a/lib/active_model/serializer.rb b/lib/active_model/serializer.rb
index dddb465c..8409cf81 100644
--- a/lib/active_model/serializer.rb
+++ b/lib/active_model/serializer.rb
@@ -347,7 +347,7 @@ module ActiveModel
       return Enumerator.new {} unless object
 
       Enumerator.new do |y|
-        self.class._reflections.each do |key, reflection|
+        (self.instance_reflections ||= self.class._reflections.deep_dup).each do |key, reflection|
           next if reflection.excluded?(self)
           next unless include_directive.key?(key)
 
@@ -411,6 +411,6 @@ module ActiveModel
 
     protected
 
-    attr_accessor :instance_options
+    attr_accessor :instance_options, :instance_reflections
   end
 end
diff --git a/lib/active_model/serializer/reflection.rb b/lib/active_model/serializer/reflection.rb
index a64aa3c4..6353933e 100644
--- a/lib/active_model/serializer/reflection.rb
+++ b/lib/active_model/serializer/reflection.rb
@@ -151,6 +151,9 @@ module ActiveModel
       # @yield [ActiveModel::Serializer]
       # @return [:nil, associated resource or resource collection]
       def value(serializer, include_slice)
+        # NOTE(BF): This method isn't thread-safe because the _reflections class attribute is not thread-safe
+        # Therefore, when we build associations from reflections, we dup the entire reflection instance.
+        # Better solutions much appreciated!
         @object = serializer.object
         @scope = serializer.scope
 
diff --git a/test/serializers/reflection_test.rb b/test/serializers/reflection_test.rb
index 11cb154b..bac52deb 100644
--- a/test/serializers/reflection_test.rb
+++ b/test/serializers/reflection_test.rb
@@ -423,5 +423,57 @@ module ActiveModel
       end
       # rubocop:enable Metrics/AbcSize
     end
+    class ThreadedReflectionTest < ActiveSupport::TestCase
+      class Post < ::Model
+        attributes :id, :title, :body
+        associations :comments
+      end
+      class Comment < ::Model
+        attributes :id, :body
+        associations :post
+      end
+      class CommentSerializer < ActiveModel::Serializer
+        type 'comment'
+        attributes :id, :body
+        has_one :post
+      end
+      class PostSerializer < ActiveModel::Serializer
+        type 'post'
+        attributes :id, :title, :body
+        has_many :comments, serializer: CommentSerializer do
+          sleep 0.1
+          object.comments
+        end
+      end
+
+      # per https://github.com/rails-api/active_model_serializers/issues/2270
+      def test_concurrent_serialization
+        post1 = Post.new(id: 1, title: 'Post 1 Title', body: 'Post 1 Body')
+        post1.comments = [Comment.new(id: 1, body: 'Comment on Post 1', post: post1)]
+        post2 = Post.new(id: 2, title: 'Post 2 Title', body: 'Post 2 Body')
+        post2.comments = [Comment.new(id: 2, body: 'Comment on Post 2', post: post2)]
+        serialized_posts = {
+          first: Set.new,
+          second: Set.new
+        }
+        t1 = Thread.new do
+          10.times do
+            serialized_posts[:first] << PostSerializer.new(post1, {}).to_json
+          end
+        end
+        t2 = Thread.new do
+          10.times do
+            serialized_posts[:second] << PostSerializer.new(post2, {}).to_json
+          end
+        end
+        t1.join
+        t2.join
+        expected_first_post_serialization  = '{"id":1,"title":"Post 1 Title","body":"Post 1 Body","comments":[{"id":1,"body":"Comment on Post 1"}]}'
+        expected_second_post_serialization = '{"id":2,"title":"Post 2 Title","body":"Post 2 Body","comments":[{"id":2,"body":"Comment on Post 2"}]}'
+
+        assert_equal [expected_second_post_serialization], serialized_posts[:second].to_a
+        assert_equal [expected_first_post_serialization], serialized_posts[:first].to_a
+      end
+    end
   end
 end