diff --git src/wp-admin/edit-form-blocks.php src/wp-admin/edit-form-blocks.php
index d998a11dad..66355e0b67 100644
--- src/wp-admin/edit-form-blocks.php
+++ src/wp-admin/edit-form-blocks.php
@@ -356,6 +356,16 @@ wp_enqueue_style( 'wp-edit-post' );
 do_action( 'enqueue_block_editor_assets' );
 
 require_once( ABSPATH . 'wp-admin/admin-header.php' );
+
+/**
+ * Fires after all built-in meta boxes have been added.
+ *
+ * @since 3.0.0
+ *
+ * @param string  $post_type Post type.
+ * @param WP_Post $post      Post object.
+ */
+do_action( 'add_meta_boxes', $post_type, $post );
 ?>
 
 <div class="block-editor">