diff --git a/actionpack/lib/action_dispatch/routing/polymorphic_routes.rb b/actionpack/lib/action_dispatch/routing/polymorphic_routes.rb
index 517936bc96..e8ba1de40e 100644
--- a/actionpack/lib/action_dispatch/routing/polymorphic_routes.rb
+++ b/actionpack/lib/action_dispatch/routing/polymorphic_routes.rb
@@ -42,6 +42,18 @@ module ActionDispatch
     #
     #   edit_polymorphic_path(@post)              # => "/posts/1/edit"
     #   polymorphic_path(@post, :format => :pdf)  # => "/posts/1.pdf"
+    #
+    # == Using with mounted engines
+    #
+    # If you use mounted engine, there is a possibility that you will need to use
+    # polymorphic_url pointing at engine's routes. To do that, just pass proxy used
+    # to reach engine's routes as a first argument:
+    #
+    # For example:
+    #
+    # polymorphic_url([blog, @post])  # it will call blog.post_path(@post)
+    # form_for([blog, @post])         # => "/blog/posts/1
+    #
     module PolymorphicRoutes
       # Constructs a call to a named RESTful route for the given record and returns the
       # resulting URL string. For example:
@@ -78,6 +90,9 @@ module ActionDispatch
       def polymorphic_url(record_or_hash_or_array, options = {})
         if record_or_hash_or_array.kind_of?(Array)
           record_or_hash_or_array = record_or_hash_or_array.compact
+          if record_or_hash_or_array.first.is_a?(ActionDispatch::Routing::RoutesProxy)
+            proxy = record_or_hash_or_array.shift
+          end
           record_or_hash_or_array = record_or_hash_or_array[0] if record_or_hash_or_array.size == 1
         end
 
@@ -111,7 +126,11 @@ module ActionDispatch
           args.last.kind_of?(Hash) ? args.last.merge!(url_options) : args << url_options
         end
 
-        url_for _routes.url_helpers.__send__("hash_for_#{named_route}", *args)
+        if proxy
+          proxy.send(named_route, *args)
+        else
+          url_for _routes.url_helpers.__send__("hash_for_#{named_route}", *args)
+        end
       end
 
       # Returns the path component of a URL for the given record. It uses
diff --git a/actionpack/test/activerecord/polymorphic_routes_test.rb b/actionpack/test/activerecord/polymorphic_routes_test.rb
index 94b8a8c7c3..23c9fb839e 100644
--- a/actionpack/test/activerecord/polymorphic_routes_test.rb
+++ b/actionpack/test/activerecord/polymorphic_routes_test.rb
@@ -57,6 +57,14 @@ class PolymorphicRoutesTest < ActionController::TestCase
     @blog_blog = Blog::Blog.new
   end
 
+  def test_passing_routes_proxy
+    with_namespaced_routes(:blog) do
+      proxy = ActionDispatch::Routing::RoutesProxy.new(_routes, self)
+      @blog_post.save
+      assert_equal "http://example.com/posts/#{@blog_post.id}", polymorphic_url([proxy, @blog_post])
+    end
+  end
+
   def test_namespaced_model
     with_namespaced_routes(:blog) do
       @blog_post.save
diff --git a/railties/test/railties/mounted_engine_test.rb b/railties/test/railties/mounted_engine_test.rb
index be5bb30a9a..1df6326d26 100644
--- a/railties/test/railties/mounted_engine_test.rb
+++ b/railties/test/railties/mounted_engine_test.rb
@@ -18,6 +18,7 @@ module ApplicationTests
           match "/engine_route" => "application_generating#engine_route"
           match "/engine_route_in_view" => "application_generating#engine_route_in_view"
           match "/url_for_engine_route" => "application_generating#url_for_engine_route"
+          match "/polymorphic_route" => "application_generating#polymorphic_route"
           scope "/:user", :user => "anonymous" do
             mount Blog::Engine => "/blog"
           end
@@ -25,6 +26,26 @@ module ApplicationTests
         end
       RUBY
 
+      @plugin.write "app/models/blog/post.rb", <<-RUBY
+        module Blog
+          class Post
+            extend ActiveModel::Naming
+
+            def id
+              44
+            end
+
+            def to_param
+              id.to_s
+            end
+
+            def new_record?
+              false
+            end
+          end
+        end
+      RUBY
+
       @plugin.write "lib/blog.rb", <<-RUBY
         module Blog
           class Engine < ::Rails::Engine
@@ -78,6 +99,10 @@ module ApplicationTests
           def url_for_engine_route
             render :text => blog.url_for(:controller => "blog/posts", :action => "index", :user => "john", :only_path => true)
           end
+
+          def polymorphic_route
+            render :text => polymorphic_url([blog, Blog::Post.new])
+          end
         end
       RUBY
 
@@ -141,6 +166,11 @@ module ApplicationTests
       script_name "/foo"
       get "/someone/blog/generate_application_route", {}, 'SCRIPT_NAME' => '/foo'
       assert_equal "/foo/", last_response.body
+      reset_script_name!
+
+      # test polymorphic routes
+      get "/polymorphic_route"
+      assert_equal "http://example.org/anonymous/blog/posts/44", last_response.body
     end
   end
 end