diff --git a/wp-includes/default-filters.php b/wp-includes/default-filters.php
index fda548ed54..6e98a069fd 100644
--- a/wp-includes/default-filters.php
+++ b/wp-includes/default-filters.php
@@ -514,6 +514,7 @@ add_action( 'init', 'wp_sitemaps_get_server' );
  */
 // Theme.
 add_action( 'setup_theme', 'create_initial_theme_features', 0 );
+add_action( 'setup_theme', '_add_default_theme_supports', 1 );
 add_action( 'wp_loaded', '_custom_header_background_just_in_time' );
 add_action( 'wp_head', '_custom_logo_header_styles' );
 add_action( 'plugins_loaded', '_wp_customize_include' );
diff --git a/wp-includes/theme.php b/wp-includes/theme.php
index e4256f00a1..351d7c8546 100644
--- a/wp-includes/theme.php
+++ b/wp-includes/theme.php
@@ -4164,3 +4164,20 @@ function create_initial_theme_features() {
 function wp_is_block_theme() {
 	return wp_get_theme()->is_block_theme();
 }
+
+/**
+ * Adds default theme supports for block themes when the 'setup_theme' action fires.
+ *
+ * @since 5.9.0
+ * @access private
+ */
+function _add_default_theme_supports() {
+	if ( wp_is_block_theme() ) {
+		add_theme_support( 'post-thumbnails' );
+		add_theme_support( 'responsive-embeds' );
+		add_theme_support( 'editor-styles' );
+		add_theme_support( 'html5', array( 'comment-form', 'comment-list', 'style', 'script' ) );
+		add_theme_support( 'automatic-feed-links' );
+		add_filter( 'should_load_separate_core_block_assets', '__return_true' );
+	}
+}
diff --git a/wp-includes/version.php b/wp-includes/version.php
index 69701c2600..65c5c4c81c 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.9-beta2-52368';
+$wp_version = '5.9-beta2-52369';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.