The Slaty-backed Gull is a very large gull at 27 inches (68.5 centimeters) long. It has a white head, belly, and tail with a dark slaty-gray back and wings with a broad white trailing edge. The wings and back are slightly darker than those of the Western Gull. The underside of the wings features a "string of pearls" pattern along their edges; these can be seen from below when the bird is in flight.
Its eyes are yellow. The legs are pink and short when compared with those of similar-looking gulls, and the body appears more stout. The bill is yellow with orange-red subterminal spot. Immature gulls' plumage is brown, similar to the that of the Great Black-backed Gull, but paler, and is practically indistinguishable from the immature Herring Gull in the field.
Like other gulls, the Slaty-backed Gull is both a forager and a scavenger.