diff --git src/wp-includes/nav-menu-template.php src/wp-includes/nav-menu-template.php
index 8895476..36cc8f4 100644
--- src/wp-includes/nav-menu-template.php
+++ src/wp-includes/nav-menu-template.php
@@ -431,6 +431,23 @@ function _wp_menu_item_classes_by_context( &$menu_items ) {
 				$classes[] = 'menu-item-home';
 		}
 
+		// Get an array of all public custom post types.
+		$post_types = get_post_types( array(
+			'public'   => true,
+			'_builtin' => false
+		) );
+
+		// Check if a custom post type single item is being viewed.
+		if ( is_singular( $post_types ) ) {
+
+			// Get the post type being viewed.
+			$post_type = get_post_type();
+
+			if ( $post_type === $menu_item->object ) {
+				$classes[] = 'current_page_parent';
+			}
+		}
+
 		// back-compat with wp_page_menu: add "current_page_parent" to static home page link for any non-page query
 		if ( ! empty( $home_page_id ) && 'post_type' == $menu_item->type && empty( $wp_query->is_page ) && $home_page_id == $menu_item->object_id )
 			$classes[] = 'current_page_parent';