diff --git a/components/remote_cocoa/app_shim/native_widget_ns_window_bridge.mm b/components/remote_cocoa/app_shim/native_widget_ns_window_bridge.mm
index 6e0a50325d731..e53c6c9749f73 100644
--- a/components/remote_cocoa/app_shim/native_widget_ns_window_bridge.mm
+++ b/components/remote_cocoa/app_shim/native_widget_ns_window_bridge.mm
@@ -1637,7 +1637,15 @@ void NativeWidgetNSWindowBridge::FullscreenControllerTransitionComplete(
   UpdateWindowDisplay();
 
   // Add any children that were skipped during the fullscreen transition.
+  // OrderChildren() may call -[NSWindow beginSheet:completionHandler:] for a
+  // deferred window-modal sheet child, which spins a nested run-loop that can
+  // close this window and free `this`. Guard with a WeakPtr, mirroring the fix
+  // in FullscreenControllerToggleFullscreen() (https://crbug.com/503792787).
+  auto weak_ptr = factory_.GetWeakPtr();
   OrderChildren();
+  if (!weak_ptr) {
+    return;
+  }
 
   host_->OnWindowFullscreenTransitionComplete(is_fullscreen);
   if (is_fullscreen && immersive_mode_controller_) {