The lesser scaup measures 15 to 19 inches. The breeding male has a blackish head, black chest, and brownish-black tail and rump. The abdomen is white, the bill pale blue with a black tip, and the eyes are bright yellow. The head is pointed toward the rear of the crown. The female has dark brown upper parts and a gray-brown breast and flanks. The abdomen is white and the pale blue bill has a dark tip. The peaked brown head has a small white patch at the base of the bill in the winter. In flight both sexes show a white wing stripe on the speculum, and dull, dark gray primaries.