diff --git a/actionpack/CHANGELOG.md b/actionpack/CHANGELOG.md
index 0e6404992d..f3cf69d758 100644
--- a/actionpack/CHANGELOG.md
+++ b/actionpack/CHANGELOG.md
@@ -1,4 +1,11 @@
-*   Check `request.path_parameters` encoding at the point they're set
+*   Fix 'defaults' option for root route.
+
+    A regression from some refactoring for the 5.0 release, this change
+    fixes the use of 'defaults' (default parameters) in the 'root' routing method.
+
+    *Chris Arcand*
+
+*   Check `request.path_parameters` encoding at the point they're set.
 
     Check for any non-UTF8 characters in path parameters at the point they're
     set in `env`. Previously they were checked for when used to get a controller
diff --git a/actionpack/lib/action_dispatch/routing/mapper.rb b/actionpack/lib/action_dispatch/routing/mapper.rb
index 220363d1b0..06b91ee9f6 100644
--- a/actionpack/lib/action_dispatch/routing/mapper.rb
+++ b/actionpack/lib/action_dispatch/routing/mapper.rb
@@ -1932,7 +1932,14 @@ to this:
 
         def match_root_route(options)
           name = has_named_route?(:root) ? nil : :root
-          match '/', { :as => name, :via => :get }.merge!(options)
+          defaults_option = options.delete(:defaults)
+          args = ['/', { as: name, via: :get }.merge!(options)]
+
+          if defaults_option
+            defaults(defaults_option) { match(*args) }
+          else
+            match(*args)
+          end
         end
       end
 
diff --git a/actionpack/test/dispatch/routing_test.rb b/actionpack/test/dispatch/routing_test.rb
index 0aeec02751..68b2ceceaa 100644
--- a/actionpack/test/dispatch/routing_test.rb
+++ b/actionpack/test/dispatch/routing_test.rb
@@ -1788,6 +1788,24 @@ class TestRoutingMapper < ActionDispatch::IntegrationTest
     assert_equal 1, @request.params[:page]
   end
 
+  def test_keyed_default_string_params_with_root
+    draw do
+      root to: 'pages#show', defaults: { id: 'home' }
+    end
+
+    get '/'
+    assert_equal 'home', @request.params[:id]
+  end
+
+  def test_default_string_params_with_root
+    draw do
+      root to: 'pages#show', id: 'home'
+    end
+
+    get '/'
+    assert_equal 'home', @request.params[:id]
+  end
+
   def test_resource_constraints
     draw do
       resources :products, :constraints => { :id => /\d{4}/ } do