The Spot-billed Duck is a large duck with a mottled dark-brown back, a blackish rump and undertail, a light buff face and a dark brown cap. Dark brown lines run through and below the eye, and the neck is light in front and dark behind. The bill is black with a distinct yellow tip or spot. The terminal half of the inner tertial feathers is white, showing in the resting bird as a distinct white patch on the dark body, and in flight it shows a blue speculum. The legs are bright reddish orange.