diff --git a/ui/android/overscroll_refresh.h b/ui/android/overscroll_refresh.h
index aaaaaaaaaa..bbbbbbbbbb 100644
--- a/ui/android/overscroll_refresh.h
+++ b/ui/android/overscroll_refresh.h
@@ -124,9 +124,12 @@ class UI_ANDROID_EXPORT OverscrollRefresh {
     kEnabled,
   } scroll_consumption_state_;

-  float viewport_width_;
-  float scroll_begin_x_;
-  float scroll_begin_y_;
+  // Initialize to zero so a renderer-triggered OnOverscrolled() that arrives
+  // before the first OnFrameUpdated() (i.e. before any RenderFrameMetadata)
+  // does not read uninitialized heap bytes at the in_x_direction branch.
+  float viewport_width_ = 0.f;
+  float scroll_begin_x_ = 0.f;
+  float scroll_begin_y_ = 0.f;
   const float edge_width_;  // in px
   const raw_ptr<OverscrollRefreshHandler, DanglingUntriaged> handler_;
   bool touchpad_overscroll_history_navigation_enabled_ = false;