白秋沙鸭（Mergellus albellus），或简称白秋沙，又名斑头秋沙鸭，是一种细小的鸭，介乎秋沙鸭属及鹊鸭属之间，会与鹊鸭混种。牠们是白秋沙鸭属下的唯一种。

由于雄性白秋沙鸭的雪白外表，飞行时黑白分明，故易于辨别。雌性及雏鸭胸部、前额及顶冠则呈灰色，很易与棕硬尾鸭混淆。白秋沙鸭的翼上有白圆点。鸟喙呈钩状及有锯齿，可以帮助捕捉鱼类。

The smew (Mergellus albellus) is a species of duck, and is the only living member of the genus Mergellus. This genus is closely related to Mergus and is sometimes included in it, though it might be closer to the goldeneyes (Bucephala). The smew has interbred with the common goldeneye (B. clangula).