diff --git a/core/modules/toolbar/css/toolbar.menu.css b/core/modules/toolbar/css/toolbar.menu.css
index c9d22d757da8..b4e8b2ef1496 100644
--- a/core/modules/toolbar/css/toolbar.menu.css
+++ b/core/modules/toolbar/css/toolbar.menu.css
@@ -13,6 +13,14 @@
   position: relative;
   width: auto;
 }
+
+/**
+ * Hidden vertical toolbar sub-menus by default.
+ */
+.toolbar .toolbar-tray-vertical .toolbar-menu ul {
+  display: none;
+}
+
 /**
  * Hidden horizontal toolbar handle icon.
  */
diff --git a/core/themes/stable/css/toolbar/toolbar.menu.css b/core/themes/stable/css/toolbar/toolbar.menu.css
index c9d22d757da8..b4e8b2ef1496 100644
--- a/core/themes/stable/css/toolbar/toolbar.menu.css
+++ b/core/themes/stable/css/toolbar/toolbar.menu.css
@@ -13,6 +13,14 @@
   position: relative;
   width: auto;
 }
+
+/**
+ * Hidden vertical toolbar sub-menus by default.
+ */
+.toolbar .toolbar-tray-vertical .toolbar-menu ul {
+  display: none;
+}
+
 /**
  * Hidden horizontal toolbar handle icon.
  */