The Red-breasted Merganser (Mergus serrator) is a diving duck.

Its breeding habitat is freshwater lakes and rivers across northern North America, Greenland, Europe and Asia. It nests in sheltered locations on the ground near water. It is migratory and many northern breeders winter in coastal waters further south.

The adult Red-breasted Merganser is 52-58 cm long with a 67-82 cm wingspan. It has a spiky crest and long thin red bill with serrated edges. Adult males have a dark head with a green sheen, a white neck with a rusty breast, a black back and white underparts. Adult females have a rusty head and a greyish body. Juveniles are like the female, but lack the white collar and have a smaller white wing patch.

The call of the female is a rasping prrak prrak, and the male gives a feeble hiccup-and-sneeze display call.

Red-breasted Mergansers dive and swim underwater. They mainly eat small fish, but also aquatic insects, crustaceans and frogs.

The Red-breasted Merganser is one of the species to which the Agreement on the Conservation of African-Eurasian Migratory Waterbirds (AEWA) applies.

The Red-breasted Merganser is one of the fastest birds in level flight.