diff --git a/src/wp-admin/includes/class-wp-theme-install-list-table.php b/src/wp-admin/includes/class-wp-theme-install-list-table.php
index 37ee8d459d..f721f9473d 100644
--- a/src/wp-admin/includes/class-wp-theme-install-list-table.php
+++ b/src/wp-admin/includes/class-wp-theme-install-list-table.php
@@ -246,15 +246,15 @@ class WP_Theme_Install_List_Table extends WP_Themes_List_Table {
 	 * @param object $theme {
 	 *     An object that contains theme data returned by the WordPress.org API.
 	 *
-	 *     @type string $name           Theme name, e.g. 'Twenty Twenty'.
-	 *     @type string $slug           Theme slug, e.g. 'twentytwenty'.
+	 *     @type string $name           Theme name, e.g. 'Twenty Twenty-One'.
+	 *     @type string $slug           Theme slug, e.g. 'twentytwentyone'.
 	 *     @type string $version        Theme version, e.g. '1.1'.
 	 *     @type string $author         Theme author username, e.g. 'melchoyce'.
-	 *     @type string $preview_url    Preview URL, e.g. 'https://2020.wordpress.net/'.
-	 *     @type string $screenshot_url Screenshot URL, e.g. 'https://wordpress.org/themes/twentytwenty/'.
+	 *     @type string $preview_url    Preview URL, e.g. 'https://2021.wordpress.net/'.
+	 *     @type string $screenshot_url Screenshot URL, e.g. 'https://wordpress.org/themes/twentytwentyone/'.
 	 *     @type float  $rating         Rating score.
 	 *     @type int    $num_ratings    The number of ratings.
-	 *     @type string $homepage       Theme homepage, e.g. 'https://wordpress.org/themes/twentytwenty/'.
+	 *     @type string $homepage       Theme homepage, e.g. 'https://wordpress.org/themes/twentytwentyone/'.
 	 *     @type string $description    Theme description.
 	 *     @type string $download_link  Theme ZIP download URL.
 	 * }
diff --git a/src/wp-admin/includes/update-core.php b/src/wp-admin/includes/update-core.php
index 27599cc192..aee5436305 100644
--- a/src/wp-admin/includes/update-core.php
+++ b/src/wp-admin/includes/update-core.php
@@ -835,6 +835,7 @@ $_new_bundled_files = array(
 	'themes/twentyseventeen/' => '4.7',
 	'themes/twentynineteen/'  => '5.0',
 	'themes/twentytwenty/'    => '5.3',
+	'themes/twentytwentyone/' => '5.6',
 );
 
 /**
diff --git a/src/wp-admin/nav-menus.php b/src/wp-admin/nav-menus.php
index 0715c854b5..adad318ee3 100644
--- a/src/wp-admin/nav-menus.php
+++ b/src/wp-admin/nav-menus.php
@@ -596,8 +596,8 @@ if ( ! $locations_screen ) : // Main tab.
 		/* translators: 1: URL to Widgets screen, 2 and 3: The names of the default themes. */
 		__( 'Menus can be displayed in locations defined by your theme, even used in sidebars by adding a &#8220;Navigation Menu&#8221; widget on the <a href="%1$s">Widgets</a> screen. If your theme does not support the navigation menus feature (the default themes, %2$s and %3$s, do), you can learn about adding this support by following the Documentation link to the side.' ),
 		admin_url( 'widgets.php' ),
-		'Twenty Nineteen',
-		'Twenty Twenty'
+		'Twenty Twenty',
+		'Twenty Twenty-One'
 	) . '</p>';
 	$overview .= '<p>' . __( 'From this screen you can:' ) . '</p>';
 	$overview .= '<ul><li>' . __( 'Create, edit, and delete menus' ) . '</li>';
diff --git a/src/wp-includes/class-wp-theme.php b/src/wp-includes/class-wp-theme.php
index 9bf81c9352..f75b957c71 100644
--- a/src/wp-includes/class-wp-theme.php
+++ b/src/wp-includes/class-wp-theme.php
@@ -59,6 +59,7 @@ final class WP_Theme implements ArrayAccess {
 		'twentyseventeen' => 'Twenty Seventeen',
 		'twentynineteen'  => 'Twenty Nineteen',
 		'twentytwenty'    => 'Twenty Twenty',
+		'twentytwentyone' => 'Twenty Twenty-One',
 	);
 
 	/**
diff --git a/src/wp-includes/default-constants.php b/src/wp-includes/default-constants.php
index bb57b96536..21a855ad25 100644
--- a/src/wp-includes/default-constants.php
+++ b/src/wp-includes/default-constants.php
@@ -409,7 +409,7 @@ function wp_templating_constants() {
 	 * @see WP_Theme::get_core_default_theme()
 	 */
 	if ( ! defined( 'WP_DEFAULT_THEME' ) ) {
-		define( 'WP_DEFAULT_THEME', 'twentytwenty' );
+		define( 'WP_DEFAULT_THEME', 'twentytwentyone' );
 	}
 
 }
diff --git a/tests/phpunit/tests/theme.php b/tests/phpunit/tests/theme.php
index 22110f352f..c4022eefcb 100644
--- a/tests/phpunit/tests/theme.php
+++ b/tests/phpunit/tests/theme.php
@@ -19,6 +19,7 @@ class Tests_Theme extends WP_UnitTestCase {
 		'twentyseventeen',
 		'twentynineteen',
 		'twentytwenty',
+		'twentytwentyone',
 	);
 
 	function setUp() {