Red-necked Phalarope is about 18 cm (7 in) in length, with lobed toes and a straight, fine bill. The breeding female is predominantly dark grey above, with a chestnut neck and upper breast, black face and white throat. The breeding male is a duller version of the female. They have lobed toes to assist with their swimming. Young birds are grey and brown above, with buff underparts and a black patch through the eye. In winter, the plumage is essentially grey above and white below, but the black eyepatch is always present. They have a sharp call described as a whit or twit.

The typical avian sex roles are reversed in the three phalarope species. Females are larger and more brightly coloured than males. The females pursue males, compete for nesting territory, and will aggressively defend their nests and chosen mates. Once the females lay their eggs, they begin their southward migration, leaving the males to incubate the eggs and look after the young. Three to seven eggs are laid in a ground nest near a marshy area. The young mainly feed themselves and are able to fly within 20 days of birth.

When feeding, a Red-necked Phalarope will often swim in a small, rapid circle, forming a small whirlpool. This behaviour is thought to aid feeding by raising food from the bottom of shallow water. The bird will reach into the center of the vortex with its bill, plucking small insects or crustaceans caught up therein. On the open ocean, they are often found where converging currents produce upwellings.