The smew (Mergellus albellus) is a species of duck, and is the only living member of the genus Mergellus. Mergellus is a diminutive of Mergus and albellus is from Latin albus "white". This genus is closely related to Mergus and is sometimes included in it, though it might be closer to the goldeneyes (Bucephala).
The drake smew, with its 'cracked ice' or 'panda' appearance, is unmistakable, and looks very black-and-white in flight. The females and immature males are grey birds with chestnut foreheads and crowns, and can be confused at a distance with the ruddy duck; they are often known as "redhead" smew. It has oval white wing-patches in flight. The smew's bill has a hooked tip and serrated edges, which help it catch fish when it dives for them.
A seaduck fossil from the Middle Miocene shows that birds similar to smew existed up to 13 million years ago. The extant species dates back to the Late Pleistocene. The smew has hybridized with the common goldeneye (B. clangula).