diff --git a/wp-admin/includes/dashboard.php b/wp-admin/includes/dashboard.php
index aad170c97c..833a617bc7 100644
--- a/wp-admin/includes/dashboard.php
+++ b/wp-admin/includes/dashboard.php
@@ -1987,21 +1987,32 @@ function wp_dashboard_empty() {}
  * Displays a welcome panel to introduce users to WordPress.
  *
  * @since 3.3.0
+ * @since 5.9.0 Send users to the Site Editor if the current theme is block-based.
  */
 function wp_welcome_panel() {
+	$customize_url          = null;
+	$can_edit_theme_options = current_user_can( 'edit_theme_options' );
+	$can_customize          = current_user_can( 'customize' );
+	$is_block_based_theme   = wp_is_block_template_theme();
+
+	if ( $is_block_based_theme && $can_edit_theme_options ) {
+		$customize_url = esc_url( admin_url( 'site-editor.php' ) );
+	} elseif ( ! $is_block_based_theme && $can_customize ) {
+		$customize_url = wp_customize_url();
+	}
 	?>
 	<div class="welcome-panel-content">
 	<h2><?php _e( 'Welcome to WordPress!' ); ?></h2>
 	<p class="about-description"><?php _e( 'We&#8217;ve assembled some links to get you started:' ); ?></p>
 	<div class="welcome-panel-column-container">
 	<div class="welcome-panel-column">
-		<?php if ( current_user_can( 'customize' ) ) : ?>
+		<?php if ( $customize_url ) : ?>
 			<h3><?php _e( 'Get Started' ); ?></h3>
-			<a class="button button-primary button-hero load-customize hide-if-no-customize" href="<?php echo wp_customize_url(); ?>"><?php _e( 'Customize Your Site' ); ?></a>
+			<a class="button button-primary button-hero load-customize hide-if-no-customize" href="<?php echo $customize_url; ?>"><?php _e( 'Customize Your Site' ); ?></a>
 		<?php endif; ?>
 		<a class="button button-primary button-hero hide-if-customize" href="<?php echo esc_url( admin_url( 'themes.php' ) ); ?>"><?php _e( 'Customize Your Site' ); ?></a>
 		<?php if ( current_user_can( 'install_themes' ) || ( current_user_can( 'switch_themes' ) && count( wp_get_themes( array( 'allowed' => true ) ) ) > 1 ) ) : ?>
-			<?php $themes_link = current_user_can( 'customize' ) ? add_query_arg( 'autofocus[panel]', 'themes', admin_url( 'customize.php' ) ) : admin_url( 'themes.php' ); ?>
+			<?php $themes_link = $can_customize && ! $is_block_based_theme ? add_query_arg( 'autofocus[panel]', 'themes', admin_url( 'customize.php' ) ) : admin_url( 'themes.php' ); ?>
 			<p class="hide-if-no-customize">
 				<?php
 					/* translators: %s: URL to Themes panel in Customizer or Themes screen. */
diff --git a/wp-admin/includes/theme.php b/wp-admin/includes/theme.php
index 56ff048928..3d1b8d74b9 100644
--- a/wp-admin/includes/theme.php
+++ b/wp-admin/includes/theme.php
@@ -701,7 +701,14 @@ function wp_prepare_themes_for_js( $themes = null ) {
 		}
 
 		$customize_action = null;
-		if ( current_user_can( 'edit_theme_options' ) && current_user_can( 'customize' ) ) {
+
+		$can_edit_theme_options = current_user_can( 'edit_theme_options' );
+		$can_customize          = current_user_can( 'customize' );
+		$is_block_based_theme   = $theme->is_block_based();
+
+		if ( $is_block_based_theme && $can_edit_theme_options ) {
+			$customize_action = esc_url( admin_url( 'site-editor.php' ) );
+		} elseif ( ! $is_block_based_theme && $can_customize && $can_edit_theme_options ) {
 			$customize_action = esc_url(
 				add_query_arg(
 					array(
diff --git a/wp-admin/theme-editor.php b/wp-admin/theme-editor.php
index 644084318b..7db3e84a8e 100644
--- a/wp-admin/theme-editor.php
+++ b/wp-admin/theme-editor.php
@@ -196,7 +196,7 @@ if ( $file_description !== $file_show ) {
 	</div>
 <?php endif; ?>
 
-<?php if ( preg_match( '/\.css$/', $file ) ) : ?>
+<?php if ( preg_match( '/\.css$/', $file ) && ! wp_is_block_template_theme() && current_user_can( 'customize' ) ) : ?>
 	<div id="message" class="notice-info notice">
 		<p><strong><?php _e( 'Did you know?' ); ?></strong></p>
 		<p>
diff --git a/wp-includes/class-wp-theme.php b/wp-includes/class-wp-theme.php
index 2e4512d033..66df8a6b67 100644
--- a/wp-includes/class-wp-theme.php
+++ b/wp-includes/class-wp-theme.php
@@ -1460,6 +1460,64 @@ final class WP_Theme implements ArrayAccess {
 		return false;
 	}
 
+	/**
+	 * Returns whether this theme is a block-based theme or not.
+	 *
+	 * @since 5.9.0
+	 *
+	 * @return bool
+	 */
+	public function is_block_based() {
+		$paths_to_index_block_template = array(
+			$this->get_file_path( '/block-templates/index.html' ),
+			$this->get_file_path( '/templates/index.html' ),
+		);
+
+		foreach ( $paths_to_index_block_template as $path_to_index_block_template ) {
+			if ( is_file( $path_to_index_block_template ) && is_readable( $path_to_index_block_template ) ) {
+				return true;
+			}
+		}
+
+		return false;
+	}
+
+	/**
+	 * Retrieves the path of a file in the theme.
+	 *
+	 * Searches in the stylesheet directory before the template directory so themes
+	 * which inherit from a parent theme can just override one file.
+	 *
+	 * @since 5.9.0
+	 *
+	 * @param string $file Optional. File to search for in the stylesheet directory.
+	 * @return string The path of the file.
+	 */
+	public function get_file_path( $file = '' ) {
+		$file = ltrim( $file, '/' );
+
+		$stylesheet_directory = $this->get_stylesheet_directory();
+		$template_directory   = $this->get_template_directory();
+
+		if ( empty( $file ) ) {
+			$path = $stylesheet_directory;
+		} elseif ( file_exists( $stylesheet_directory . '/' . $file ) ) {
+			$path = $stylesheet_directory . '/' . $file;
+		} else {
+			$path = $template_directory . '/' . $file;
+		}
+
+		/**
+		 * Filters the path to a file in the theme.
+		 *
+		 * @since 5.9.0
+		 *
+		 * @param string $path The file path.
+		 * @param string $file The requested file to search for.
+		 */
+		return apply_filters( 'theme_file_path', $path, $file );
+	}
+
 	/**
 	 * Determines the latest WordPress default theme that is installed.
 	 *
diff --git a/wp-includes/version.php b/wp-includes/version.php
index 9ede554ad4..c7d971fce6 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.9-alpha-52278';
+$wp_version = '5.9-alpha-52279';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.