diff --git a/actionpack/CHANGELOG.md b/actionpack/CHANGELOG.md
index a0a12f1b0e5dd56108a8266d0bf8f2ed8d2179a6..a50b81b43985e0c6b2fce234f6be44d8b2b14d4f 100644
--- a/actionpack/CHANGELOG.md
+++ b/actionpack/CHANGELOG.md
@@ -1,3 +1,11 @@
+*   Fix regression in mounted engine named routes generation for app deployed to
+    a subdirectory. `relative_url_root` was prepended to the path twice (e.g.
+    "/subdir/subdir/engine_path" instead of "/subdir/engine_path")
+
+    Fixes #20920. Fixes #21459.
+
+    *Matthew Erhard*
+
 *   ActionDispatch::Response#new no longer applies default headers.  If you want
     default headers applied to the response object, then call
     `ActionDispatch::Response.create`.  This change only impacts people who are
diff --git a/actionpack/lib/action_dispatch/routing/mapper.rb b/actionpack/lib/action_dispatch/routing/mapper.rb
index cfd50f616d3a51d4e583dbd43d9cbcb33bae7977..2f1c2afb9171d7ea28887cd7bc8511bb1ed844a8 100644
--- a/actionpack/lib/action_dispatch/routing/mapper.rb
+++ b/actionpack/lib/action_dispatch/routing/mapper.rb
@@ -665,6 +665,7 @@ def optimize_routes_generation?; false; end
                   super(options)
                 else
                   prefix_options = options.slice(*_route.segment_keys)
+                  prefix_options[:relative_url_root] = ''.freeze
                   # we must actually delete prefix segment keys to avoid passing them to next url_for
                   _route.segment_keys.each { |k| options.delete(k) }
                   _routes.url_helpers.send("#{name}_path", prefix_options)
diff --git a/actionpack/lib/action_dispatch/routing/route_set.rb b/actionpack/lib/action_dispatch/routing/route_set.rb
index e4b8d5993e1b7350af85111500521c35cef89776..339e2b7c4abf3d4469208ea0533ed186a3d789b5 100644
--- a/actionpack/lib/action_dispatch/routing/route_set.rb
+++ b/actionpack/lib/action_dispatch/routing/route_set.rb
@@ -656,14 +656,18 @@ def generate(route_key, options, recall = {})
 
       RESERVED_OPTIONS = [:host, :protocol, :port, :subdomain, :domain, :tld_length,
                           :trailing_slash, :anchor, :params, :only_path, :script_name,
-                          :original_script_name]
+                          :original_script_name, :relative_url_root]
 
       def optimize_routes_generation?
         default_url_options.empty?
       end
 
       def find_script_name(options)
-        options.delete(:script_name) || relative_url_root || ''
+        options.delete(:script_name) || find_relative_url_root(options) || ''
+      end
+
+      def find_relative_url_root(options)
+        options.delete(:relative_url_root) || relative_url_root
       end
 
       def path_for(options, route_name = nil)
diff --git a/railties/test/railties/engine_test.rb b/railties/test/railties/engine_test.rb
index 79bd7a8241595f1f915544a369eedb6bddf9e028..2c82f728ee6284144afc01c7b7e95399efcfd489 100644
--- a/railties/test/railties/engine_test.rb
+++ b/railties/test/railties/engine_test.rb
@@ -1304,6 +1304,55 @@ def index
       assert_equal '/foo/bukkits/bukkit', last_response.body
     end
 
+    test "paths are properly generated when application is mounted at sub-path and relative_url_root is set" do
+      add_to_config "config.relative_url_root = '/foo'"
+
+      @plugin.write "lib/bukkits.rb", <<-RUBY
+        module Bukkits
+          class Engine < ::Rails::Engine
+            isolate_namespace Bukkits
+          end
+        end
+      RUBY
+
+      app_file "app/controllers/bar_controller.rb", <<-RUBY
+        class BarController < ApplicationController
+          def index
+            render text: bukkits.bukkit_path
+          end
+        end
+      RUBY
+
+      app_file "config/routes.rb", <<-RUBY
+        Rails.application.routes.draw do
+          get '/bar' => 'bar#index', :as => 'bar'
+          mount Bukkits::Engine => "/bukkits", :as => "bukkits"
+        end
+      RUBY
+
+      @plugin.write "config/routes.rb", <<-RUBY
+        Bukkits::Engine.routes.draw do
+          get '/bukkit' => 'bukkit#index'
+        end
+      RUBY
+
+      @plugin.write "app/controllers/bukkits/bukkit_controller.rb", <<-RUBY
+        class Bukkits::BukkitController < ActionController::Base
+          def index
+            render text: main_app.bar_path
+          end
+        end
+      RUBY
+
+      boot_rails
+
+      get("/bukkits/bukkit", {}, {'SCRIPT_NAME' => '/foo'})
+      assert_equal '/foo/bar', last_response.body
+
+      get("/bar", {}, {'SCRIPT_NAME' => '/foo'})
+      assert_equal '/foo/bukkits/bukkit', last_response.body
+    end
+
   private
     def app
       Rails.application