[3] A seaduck fossil from the Middle Miocene shows that birds similar to smew existed up to 13 million years ago.

The smew was formally described in 1758 by the Swedish naturalist Carl Linnaeus in the tenth edition of his Systema Naturae under the binomial name Mergus albellus.

[6] The smew is now the only living species placed in the genus Mergellus that was introduced in 1840 by the English naturalist Prideaux Selby.

It is believed to be related to the Dutch smient ("wigeon") and the German Schmeiente or Schmünte, "wild duck.

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.

As a migrant, it leaves its breeding areas and winters on sheltered coasts or inland lakes of the Baltic Sea, the Black Sea, northern Germany and the Low Countries, with a small number reaching Great Britain (for example, at Dungeness), mostly at regular sites.

[1] An unnamed fossil seaduck, known from a humerus found in the Middle Miocene Sajóvölgyi Formation (Late Badenian, 13–12 million years ago) of Mátraszõlõs, Hungary, was assigned to Mergus.