diff --git src/wp-includes/class-wp-widget-factory.php src/wp-includes/class-wp-widget-factory.php
index 12b4553..9b9214e 100644
--- src/wp-includes/class-wp-widget-factory.php
+++ src/wp-includes/class-wp-widget-factory.php
@@ -49,24 +49,34 @@ class WP_Widget_Factory {
 	 * Registers a widget subclass.
 	 *
 	 * @since 2.8.0
+	 * @since 4.6.0 The `$widget` param can also be an instance object of `WP_Widget` instead of just a `WP_Widget` subclass name.
 	 * @access public
 	 *
-	 * @param string $widget_class The name of a WP_Widget subclass.
+	 * @param string|WP_Widget $widget Either the name of a `WP_Widget` subclass or an instance of a `WP_Widget` subclass.
 	 */
-	public function register( $widget_class ) {
-		$this->widgets[$widget_class] = new $widget_class();
+	public function register( $widget ) {
+		if ( $widget instanceof WP_Widget ) {
+			$this->widgets[ spl_object_hash( $widget ) ] = $widget;
+		} else {
+			$this->widgets[ $widget ] = new $widget();
+		}
 	}
 
 	/**
 	 * Un-registers a widget subclass.
 	 *
 	 * @since 2.8.0
+	 * @since 4.6.0 The `$widget` param can also be an instance object of `WP_Widget` instead of just a `WP_Widget` subclass name.
 	 * @access public
 	 *
-	 * @param string $widget_class The name of a WP_Widget subclass.
+	 * @param string|WP_Widget $widget Either the name of a `WP_Widget` subclass or an instance of a `WP_Widget` subclass.
 	 */
-	public function unregister( $widget_class ) {
-		unset( $this->widgets[ $widget_class ] );
+	public function unregister( $widget ) {
+		if ( $widget instanceof WP_Widget ) {
+			unset( $this->widgets[ spl_object_hash( $widget ) ] );
+		} else {
+			unset( $this->widgets[ $widget ] );
+		}
 	}
 
 	/**
diff --git tests/phpunit/tests/widgets.php tests/phpunit/tests/widgets.php
index 0ac6d92..1b6bab4 100644
--- tests/phpunit/tests/widgets.php
+++ tests/phpunit/tests/widgets.php
@@ -44,6 +44,62 @@ class Tests_Widgets extends WP_UnitTestCase {
 	}
 
 	/**
+	 * Test that registering a widget class and registering a widget instance work together.
+	 *
+	 * @see register_widget()
+	 * @see unregister_widget()
+	 * @ticket 28216
+	 */
+	function test_register_and_unregister_widget_instance() {
+		global $wp_widget_factory, $wp_registered_widgets;
+		$this->assertEmpty( $wp_widget_factory->widgets );
+		$this->assertEmpty( $wp_registered_widgets );
+
+		update_option( 'widget_search', array(
+			2 => array(
+				'title' => '',
+			),
+			'_multiwidget' => 1,
+		) );
+		update_option( 'widget_better_search', array(
+			3 => array(
+				'title' => '',
+			),
+			'_multiwidget' => 1,
+		) );
+
+		register_widget( 'WP_Widget_Search' );
+		$this->assertArrayHasKey( 'WP_Widget_Search', $wp_widget_factory->widgets );
+
+		$widget_better_search = new WP_Widget_Search();
+		$widget_better_search->id_base = 'better_search';
+		$widget_better_search->name = 'Better Search';
+		$widget_better_search->option_name = 'widget_' . $widget_better_search->id_base;
+		$widget_better_search->widget_options['classname'] = 'widget_' . $widget_better_search->id_base;
+		$widget_better_search->control_options['id_base'] = $widget_better_search->id_base;
+		register_widget( $widget_better_search );
+		$this->assertArrayHasKey( spl_object_hash( $widget_better_search ), $wp_widget_factory->widgets );
+
+		$this->assertCount( 2, $wp_widget_factory->widgets );
+		$this->assertArrayHasKey( 'WP_Widget_Search', $wp_widget_factory->widgets );
+		$this->assertArrayHasKey( spl_object_hash( $widget_better_search ), $wp_widget_factory->widgets );
+
+		$wp_widget_factory->_register_widgets();
+
+		$this->assertArrayHasKey( 'search-2', $wp_registered_widgets );
+		$this->assertArrayHasKey( 'better_search-3', $wp_registered_widgets );
+		$this->assertInstanceOf( 'WP_Widget_Search', $wp_registered_widgets['search-2']['callback'][0] );
+		$this->assertSame( $widget_better_search, $wp_registered_widgets['better_search-3']['callback'][0] );
+
+		$this->assertArrayHasKey( spl_object_hash( $widget_better_search ), $wp_widget_factory->widgets );
+		$this->assertArrayHasKey( 'WP_Widget_Search', $wp_widget_factory->widgets );
+		unregister_widget( 'WP_Widget_Search' );
+		unregister_widget( $widget_better_search );
+		$this->assertArrayNotHasKey( spl_object_hash( $widget_better_search ), $wp_widget_factory->widgets );
+		$this->assertArrayNotHasKey( 'WP_Widget_Search', $wp_widget_factory->widgets );
+	}
+
+	/**
 	 * @group sidebar
 	 */
 	function test_register_sidebars_single() {