The spotted seal is a sibling species with the harbor seal.  Their coloration is generally pale, silver gray above and below, with a darker mantle dominated by even darker oval spots of fairly uniform size (1-2 cm) and generally oriented parallel to the long axis of the body.  There may be light rings around some spots, or large irregular spots or blotches.  Spotting tends to be of fairly even distribution and darkness overall.  In harbor seals, spots are more faded and sparse on the underside.  The face and muzzle are darker.  Pups are born with a long and woolly whitish lanugo, which is shed 2-4 weeks after birth.

The dental formula of adults is I 3/2, C1/1, PC 5/5.  

Adult males are up to 1.7 m and females to 1.6 m long.  Adults weigh 82-123 kg.  At birth, spotted seals are 77-92 cm long and weigh 7-12 kg.

In addition to harbor seals, spotted seals share their range with ringed and ribbon seals.  Ribbon seals lack spots and have broad bands on a black or brown body, or as juveniles are dull gray above and lighter below, but in all age classes and both sexes ribbon seals lack the abundant spots of the spotted seal.

The range of the eastern Pacific and Western Pacific subspecies of the harbor seal overlaps with the range of the spotted seal.  Unfortunately, the consistent differences between the species are features of the skull and genetic characteristics.  Coloration, body size and shape, and size of features overlap in the two species.  Some of the best features are behavioral.  Spotted seals give birth on sea ice and usually are alone or accompanied by a male.  Harbor seals give birth in haul-out groups typical of the sites where they are found year-round.  In addition to being born in a long gray lanugo coat, spotted seals spend their first weeks on the ice, while harbor seals are generally born in a short hair coat similar to the adult, and are able to swim within hours of birth.  Where they co-occur, spotted seals give birth up to 2 months earlier than harbor seals.

Spotted seals and ringed seals can be easily confused.  Spotted seals are longer and proportionately leaner, with a longer neck, head, and muzzle.  Spotted seals rarely have many rings, whereas ringed seals have an abundance of rings and a low density of less conspicuous spots. Pups of both species are born in a long, grayish lanugo coat and would be difficult to tell apart if away from adults. However, spotted seal pups are born on top of ice floes, whereas ringed seal pups are born in lairs under snow and ice.  

Spotted seals haul-out on land, and can be found mixed in with groups of harbor seals in at least Bristol Bay, Alaska, and are said to be separable from harbor seals only by experienced observers based on behavior, response to disturbance, and subtle differences in facial features.

Spotted seals are widespread in the Sea of Okhotsk and the Sea of Japan, and reach China in the northern Yellow Sea.  They are widespread in the Bering and Chukchi Seas and range north into the Arctic Ocean north to about the edge of the continental shelf, west to about 170°E and east to the Mackenzie River Delta in Canada.  They inhabit the southern edges of the pack ice from winter to early summer. In late summer and fall, ringed seals move into coastal areas, including river mouths. They breed exclusively, and haul-out regularly, on sea ice, but do come ashore on beaches and sandbars.