diff --git a/wp-includes/block-template-utils.php b/wp-includes/block-template-utils.php
index 9b248c1453..6475ac36f2 100644
--- a/wp-includes/block-template-utils.php
+++ b/wp-includes/block-template-utils.php
@@ -20,6 +20,35 @@ if ( ! defined( 'WP_TEMPLATE_PART_AREA_UNCATEGORIZED' ) ) {
 	define( 'WP_TEMPLATE_PART_AREA_UNCATEGORIZED', 'uncategorized' );
 }
 
+/**
+ * For backward compatibility reasons,
+ * block themes might be using block-templates or block-template-parts,
+ * this function ensures we fallback to these folders properly.
+ *
+ * @since 5.9.0
+ *
+ * @param string $theme_stylesheet The stylesheet. Default is to leverage the main theme root.
+ *
+ * @return array Folder names used by block themes.
+ */
+function get_block_theme_folders( $theme_stylesheet = null ) {
+	$theme_name = null === $theme_stylesheet ? get_stylesheet() : $theme_stylesheet;
+	$root_dir   = get_theme_root( $theme_name );
+	$theme_dir  = "$root_dir/$theme_name";
+
+	if ( is_readable( $theme_dir . '/block-templates/index.html' ) ) {
+		return array(
+			'wp_template'      => 'block-templates',
+			'wp_template_part' => 'block-template-parts',
+		);
+	}
+
+	return array(
+		'wp_template'      => 'templates',
+		'wp_template_part' => 'parts',
+	);
+}
+
 /**
  * Returns a filtered list of allowed area values for template parts.
  *
@@ -224,16 +253,13 @@ function _get_block_template_file( $template_type, $slug ) {
 		return null;
 	}
 
-	$template_base_paths = array(
-		'wp_template'      => 'block-templates',
-		'wp_template_part' => 'block-template-parts',
-	);
-	$themes              = array(
+	$themes = array(
 		get_stylesheet() => get_stylesheet_directory(),
 		get_template()   => get_template_directory(),
 	);
 	foreach ( $themes as $theme_slug => $theme_dir ) {
-		$file_path = $theme_dir . '/' . $template_base_paths[ $template_type ] . '/' . $slug . '.html';
+		$template_base_paths = get_block_theme_folders( $theme_slug );
+		$file_path           = $theme_dir . '/' . $template_base_paths[ $template_type ] . '/' . $slug . '.html';
 		if ( file_exists( $file_path ) ) {
 			$new_template_item = array(
 				'slug'  => $slug,
@@ -272,17 +298,13 @@ function _get_block_templates_files( $template_type ) {
 		return null;
 	}
 
-	$template_base_paths = array(
-		'wp_template'      => 'block-templates',
-		'wp_template_part' => 'block-template-parts',
-	);
-	$themes              = array(
+	$themes         = array(
 		get_stylesheet() => get_stylesheet_directory(),
 		get_template()   => get_template_directory(),
 	);
-
 	$template_files = array();
 	foreach ( $themes as $theme_slug => $theme_dir ) {
+		$template_base_paths  = get_block_theme_folders( $theme_slug );
 		$theme_template_files = _get_block_templates_paths( $theme_dir . '/' . $template_base_paths[ $template_type ] );
 		foreach ( $theme_template_files as $template_file ) {
 			$template_base_path = $template_base_paths[ $template_type ];
diff --git a/wp-includes/theme.php b/wp-includes/theme.php
index 838fe23596..9d43d1b2a2 100644
--- a/wp-includes/theme.php
+++ b/wp-includes/theme.php
@@ -4084,5 +4084,6 @@ function create_initial_theme_features() {
  * @return boolean Whether the current theme is a block-based theme or not.
  */
 function wp_is_block_template_theme() {
-	return is_readable( get_theme_file_path( '/block-templates/index.html' ) );
+	return is_readable( get_theme_file_path( '/block-templates/index.html' ) ) ||
+		is_readable( get_theme_file_path( '/templates/index.html' ) );
 }
diff --git a/wp-includes/version.php b/wp-includes/version.php
index 59c2be791d..b6a5ab7305 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.9-alpha-52246';
+$wp_version = '5.9-alpha-52247';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.