diff --git a/actionpack/lib/action_dispatch/http/parameters.rb b/actionpack/lib/action_dispatch/http/parameters.rb
index d80d1d714c20b4b5f3f8518d9bf692ee663bd527..ae875eb830a2b5b9a52fa43c7e07e7ae2da81aef 100644
--- a/actionpack/lib/action_dispatch/http/parameters.rb
+++ b/actionpack/lib/action_dispatch/http/parameters.rb
@@ -57,7 +57,7 @@ def parameters
                    query_parameters.dup
                  end
         params.merge!(path_parameters)
-        params = set_binary_encoding(params)
+        params = set_binary_encoding(params, params[:controller], params[:action])
         set_header("action_dispatch.request.parameters", params)
         params
       end
@@ -66,6 +66,7 @@ def parameters
       def path_parameters=(parameters) #:nodoc:
         delete_header("action_dispatch.request.parameters")
 
+        parameters = set_binary_encoding(parameters, parameters[:controller], parameters[:action])
         # If any of the path parameters has an invalid encoding then
         # raise since it's likely to trigger errors further on.
         Request::Utils.check_param_encoding(parameters)
@@ -85,9 +86,10 @@ def path_parameters
 
       private
 
-        def set_binary_encoding(params)
-          action = params[:action]
-          if binary_params_for?(action)
+        def set_binary_encoding(params, controller, action)
+          return params unless controller && controller.valid_encoding?
+
+          if binary_params_for?(controller, action)
             ActionDispatch::Request::Utils.each_param_value(params) do |param|
               param.force_encoding ::Encoding::ASCII_8BIT
             end
@@ -95,8 +97,8 @@ def set_binary_encoding(params)
           params
         end
 
-        def binary_params_for?(action)
-          controller_class.binary_params_for?(action)
+        def binary_params_for?(controller, action)
+          controller_class_for(controller).binary_params_for?(action)
         rescue NameError
           false
         end
diff --git a/actionpack/lib/action_dispatch/http/request.rb b/actionpack/lib/action_dispatch/http/request.rb
index 94b0d4880cb0a0b31c7f70708024b42049b11539..0b38ab7afb5685ae2d7cef0fb88d26ed3c0b9498 100644
--- a/actionpack/lib/action_dispatch/http/request.rb
+++ b/actionpack/lib/action_dispatch/http/request.rb
@@ -76,10 +76,13 @@ def self.binary_params_for?(action); false; end
 
     def controller_class
       params = path_parameters
+      params[:action] ||= "index"
+      controller_class_for(params[:controller])
+    end
 
-      if params.key?(:controller)
-        controller_param = params[:controller].underscore
-        params[:action] ||= "index"
+    def controller_class_for(name)
+      if name
+        controller_param = name.underscore
         const_name = "#{controller_param.camelize}Controller"
         ActiveSupport::Dependencies.constantize(const_name)
       else
diff --git a/actionpack/lib/action_dispatch/journey/router.rb b/actionpack/lib/action_dispatch/journey/router.rb
index 809bfcbe8ad1bddc4b9490d8652b877db77f4562..9987a9bfa1251c53d3e44ecb337ba03f6e20f85e 100644
--- a/actionpack/lib/action_dispatch/journey/router.rb
+++ b/actionpack/lib/action_dispatch/journey/router.rb
@@ -43,6 +43,10 @@ def serve(req)
             req.path_info = "/" + req.path_info unless req.path_info.start_with? "/"
           end
 
+          parameters = route.defaults.merge parameters.transform_values { |val|
+            val.dup.force_encoding(::Encoding::UTF_8)
+          }
+
           req.path_parameters = set_params.merge parameters
 
           status, headers, body = route.app.serve(req)
@@ -67,6 +71,7 @@ def recognize(rails_req)
             rails_req.path_info   = match.post_match.sub(/^([^\/])/, '/\1')
           end
 
+          parameters = route.defaults.merge parameters
           yield(route, parameters)
         end
       end
@@ -119,7 +124,7 @@ def find_routes(req)
 
           routes.map! { |r|
             match_data = r.path.match(req.path_info)
-            path_parameters = r.defaults.dup
+            path_parameters = {}
             match_data.names.zip(match_data.captures) { |name, val|
               path_parameters[name.to_sym] = Utils.unescape_uri(val) if val
             }
diff --git a/actionpack/test/controller/routing_test.rb b/actionpack/test/controller/routing_test.rb
index fefb84e095d00b7bb1861de874d72dd01a4ed4ec..f09051b306e481b507e9292d0440e113cc89c0b0 100644
--- a/actionpack/test/controller/routing_test.rb
+++ b/actionpack/test/controller/routing_test.rb
@@ -96,6 +96,22 @@ def test_symbols_with_dashes
     assert_equal({ "artist" => "journey", "song" => "faithfully" }, hash)
   end
 
+  def test_id_encoding
+    rs.draw do
+      get "/journey/:id", to: lambda { |env|
+        param = ActionDispatch::Request.new(env).path_parameters
+        resp = ActiveSupport::JSON.encode param
+        [200, {}, [resp]]
+      }
+    end
+
+    # The encoding of the URL in production is *binary*, so we add a
+    # .b here.
+    hash = ActiveSupport::JSON.decode get(URI("http://example.org/journey/%E5%A4%AA%E9%83%8E".b))
+    assert_equal({ "id" => "太郎" }, hash)
+    assert_equal ::Encoding::UTF_8, hash["id"].encoding
+  end
+
   def test_id_with_dash
     rs.draw do
       get "/journey/:id", to: lambda { |env|
diff --git a/actionpack/test/dispatch/routing_test.rb b/actionpack/test/dispatch/routing_test.rb
index 1af0edc1d3de8dd34f2c37c0e10d220e5fb7c19d..2bb814e5f453750c6fd285c5b69c65cc033dd45e 100644
--- a/actionpack/test/dispatch/routing_test.rb
+++ b/actionpack/test/dispatch/routing_test.rb
@@ -4416,6 +4416,10 @@ def app; APP end
 
 class TestInvalidUrls < ActionDispatch::IntegrationTest
   class FooController < ActionController::Base
+    def self.binary_params_for?(action)
+      action == "show"
+    end
+
     def show
       render plain: "foo#show"
     end
@@ -4437,19 +4441,19 @@ def show
       end
 
       get "/%E2%EF%BF%BD%A6"
-      assert_response :not_found
+      assert_response :bad_request
 
       get "/foo/%E2%EF%BF%BD%A6"
-      assert_response :not_found
+      assert_response :bad_request
 
       get "/foo/show/%E2%EF%BF%BD%A6"
       assert_response :ok
 
       get "/bar/%E2%EF%BF%BD%A6"
-      assert_response :redirect
+      assert_response :bad_request
 
       get "/foobar/%E2%EF%BF%BD%A6"
-      assert_response :ok
+      assert_response :bad_request
     end
   end
 end