The Smew (Mergellus albellus) is a medium-sized bird in the wild duck family. It is a diving duck. It is a sea duck, living in marine (saltwater) locations, but it breeds in freshwater lakes. It is also known as the Redhead Smew and the White Nun. It is related to the Common Goldeneye (Bucephala clangula). 

The male and female Smew look different. The male Smew is mostly white, with a black patch around his eye, a black nape, white crest feathers, and a grey rump and sides, with two fine black lines on each side of his chest. Its beak is grey. Its legs and webbed feet are grey.

The female has a rufous-brown (red-brown) head, nape and neck, a white throat and white cheeks, with grey upperparts and a mottled-grey chest. She has brown eyes, grey beak, grey legs, and grey webbed feet. Both the male and the female have a beak with a hooked tip.