The kelp gull superficially resembles two gulls from further north in the
Atlantic Ocean, the
lesser black-backed gull and the
great black-backed gull, and is intermediate in size between these two species. This species ranges from in total length, from in wingspan and from in weight. Adult males and females weigh on average and respectively. Among standard measurements, the
wing chord is , the
bill is and the
tarsus is . The adult kelp gull has black upper parts and wings. The head, underparts, tail, and the small "mirrors" at the wing tips are white. The bill is yellow with a red spot, and the legs are greenish-yellow (brighter and yellower when breeding, duller and greener when not breeding). The call is a strident
ki-och. Juveniles have dull legs, a black bill, a dark band in the tail, and an overall grey-brown
plumage densely edged whitish, but they rapidly get a pale base to the bill and largely white head and underparts. They take three or four years to reach maturity. ==Subspecies==