diff --git a/actionview/CHANGELOG.md b/actionview/CHANGELOG.md
index 6b9c76eb29..53142a835e 100644
--- a/actionview/CHANGELOG.md
+++ b/actionview/CHANGELOG.md
@@ -1,3 +1,10 @@
+*   Fix `form_for` when both `namespace` and `as` options are present
+
+    `as` option no longer overwrites `namespace` option when generating
+    html id attribute of the form element
+
+    *Adam Niedzielski*
+
 *   Fix `excerpt` when `:separator` is `nil`.
 
     *Paul Nikitochkin*
diff --git a/actionview/lib/action_view/helpers/form_helper.rb b/actionview/lib/action_view/helpers/form_helper.rb
index 8a4830d887..ead7871fc5 100644
--- a/actionview/lib/action_view/helpers/form_helper.rb
+++ b/actionview/lib/action_view/helpers/form_helper.rb
@@ -442,10 +442,11 @@ module ActionView
         object = convert_to_model(object)
 
         as = options[:as]
+        namespace = options[:namespace]
         action, method = object.respond_to?(:persisted?) && object.persisted? ? [:edit, :patch] : [:new, :post]
         options[:html].reverse_merge!(
           class:  as ? "#{action}_#{as}" : dom_class(object, action),
-          id:     as ? "#{action}_#{as}" : [options[:namespace], dom_id(object, action)].compact.join("_").presence,
+          id:     (as ? [namespace, action, as] : [namespace, dom_id(object, action)]).compact.join("_").presence,
           method: method
         )
 
diff --git a/actionview/test/template/form_helper_test.rb b/actionview/test/template/form_helper_test.rb
index 8cca43d7ca..944884c9dd 100644
--- a/actionview/test/template/form_helper_test.rb
+++ b/actionview/test/template/form_helper_test.rb
@@ -1681,6 +1681,18 @@ class FormHelperTest < ActionView::TestCase
     assert_dom_equal expected, output_buffer
   end
 
+  def test_form_for_with_namespace_and_as_option
+    form_for(@post, namespace: 'namespace', as: 'custom_name') do |f|
+      concat f.text_field(:title)
+    end
+
+    expected = whole_form('/posts/123', 'namespace_edit_custom_name', 'edit_custom_name', method: 'patch') do
+      "<input id='namespace_custom_name_title' name='custom_name[title]' type='text' value='Hello World' />"
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
   def test_two_form_for_with_namespace
     form_for(@post, namespace: 'namespace_1') do |f|
       concat f.label(:title)