diff --git a/olivero.theme b/olivero.theme
index 2c56fd2..82f4a72 100644
--- a/olivero.theme
+++ b/olivero.theme
@@ -148,6 +148,11 @@ function olivero_preprocess_block(&$variables) {
       $variables['attributes']['class'][] = 'site-branding--bg-' . $site_branding_color;
     }
   }
+
+  // Add a primary-nav class to main menu navigation block.
+  if ($variables['plugin_id'] === 'system_menu_block:main') {
+    $variables['attributes']['class'][] = 'primary-nav';
+  }
 }
 
 /**