This gull is 37–44 cm long with a 94–110 cm wingspan and weighs from 190g to 400 grams.

The summer adult has a chocolate-brown head (not black, although it looks black from a distance), pale grey body, black tips to the primary wing feathers, and red bill and legs. The hood is lost in winter, leaving just two dark spots. Immature birds have a mottled pattern of brown spots over most of the body, and a black band on the tail. There is no difference in plumage between the sexes.

The black-headed gull is a bold and opportunistic feeder. It eats insects, fish, seeds, worms, or human food scraps. It is a noisy species, especially in colonies, with a familiar "kree-ar" call.