This dabbling duck is 42–52 cm (17–20 in) long with a 71–80 cm (28–31 in) wingspan, and a weight of 500–1,073 g (1.102–2.366 lb).[5][6] The breeding male has grey flanks and back, with a black rear end, a dark green speculum and a brilliant white patch on upper wings, obvious in flight or at rest. It has a pink breast, white belly, and a chestnut head with a creamy crown.[7] In non-breeding (eclipse) plumage, the drake looks more like the female. The female is light brown, with plumage much like a female American wigeon. It can be distinguished from most other ducks, apart from American wigeon, on shape. However, that species has a paler head and white axillaries on its underwing. The female can be a rufous morph with a redder head, and a gray morph with a more gray head.[7]
A cladistic analysis based on several morphological characters placed the surf scoter as a monotypic taxon, closest to the white-winged scoter (Melanitta deglandi) and the velvet scoter (Melanitta fusca), which are both sister taxa.[6][8] These three species form the subgenus Melanitta, distinct from the subgenus Oidemia, which contains the black scoter (Melanitta americana) and the common scoter (Melanitta nigra).
The only extinct Melanitta species, M. cerutti, used to be present in California during the late Pliocene, but it has been moved in the genus Histrionicus (Harlequin duck).[9]
The genus Melanitta is part of the Mergini tribe, a monophyletic group of the Northern Hemisphere.[8] It includes eiders, mergansers, goldeneyes and other sea ducks.[8][10] This tribe is part of the family Anatidae, along with the swans and geese.