The lesser black-backed gull is smaller than the European herring gull. The taxonomy of the herring gull / lesser black-backed gull complex is very complicated; different authorities recognize between two and eight species. This group has a ring distribution around the northern hemisphere. Differences between adjacent forms in this ring are fairly small, but by the time the circuit is completed, the end members, herring gull and lesser black-backed gull, are clearly different species. The lesser black-backed gull measures 51–64 cm (20–25 in), 124–150 cm (49–59 in) across the wings and weighs 452–1,100 g (0.996–2.425 lb), with the nominate race averaging slightly smaller than the other two subspecies. Males, at average weight of 824 g (1.817 lb), are slightly larger than females, at an average of 708 g (1.561 lb). Among standard measurements, the wing chord is 38.3 to 45 cm (15.1 to 17.7 in), the bill is 4.2 to 5.8 cm (1.7 to 2.3 in) and the tarsus is 5.2 to 6.9 cm (2.0 to 2.7 in). A confusable species is the great black-backed gull. The lesser is a much smaller bird, with slimmer build, yellow rather than pinkish legs, and smaller white "mirrors" at the wing tips. The adults have black or dark grey wings (depending on race) and back. The bill is yellow with a red spot which young peck at, inducing feeding (see fixed action pattern). The head is grayer in winter, unlike great black-backed. Annual molt for adults begins between May and August and is not complete on some birds until November. Partial pre-breeding molt between January and April
Young birds have scaly black-brown upperparts and a neat wing pattern. They take four years to reach maturity. Identification from juvenile herring gulls is most readily done by the more solidly dark (unbarred) tertial feathers.
The call is a "laughing" cry like that of the herring gull (to which this species is closely related), but with a markedly deeper pitch.