Guadalupe fur seal are sexually dimorphic, with males 1.5-2 times longer and approximately 3–4 times heavier than adult females.  They have thick pelage, with dense underfur.   Adults have moderate length whitish-cream vibrissae and long prominent ear pinnae.

Adult males have a slightly-rounded crown with the apex above the ear pinnae.  Longer, more erect, mane guard hairs on the head accentuate the minor elevation of the crown, and contribute to a slight sloping forehead at the eyes, where the longer fur ends at the shorter, more swept-back, fur of the muzzle.  The muzzle is long, narrow, straight, and tapers in width and thickness to a pointed end.  The naked black skin area of the nose, the rhinarium, is bulbous and the nares are angled downward.  The enlarged rhinaruim makes the muzzle seem slightly up-turned at the end, especially in males. A mane of longer and coarser guard hairs extends from the crown and nape to the shoulders on the back, and includes the sides and front of the neck.  The neck, chest and shoulders to the fore flippers are enlarged, and on the back can seem bi-lobed.  The abdomen is thin in comparison, and tapers to the insertion of the hind flippers. The canine teeth of adult males are larger and thicker than those of females.

Adult females, subadults, and juveniles also have a long muzzle with downward-pointing nares, but the nose is not enlarged.  The crown is flatter and the mane absent, and as a result they have a flat-topped head.  The ear pinnae are long and stand-out from the head enough to make them conspicuous.

The fore flippers have a dark, sparse, short fur that extends beyond the wrist onto the middle of the dorsal surface of the flipper in a “V” pattern that does not reach the rounded tip.  The rest of the dorsal surface, and the palms of both fore flippers are covered with a hairless black leathery skin.  The first digit is the longest, widest and thickest, and curves posteriorly, giving the flipper a swept-back look.  Digits 2-5 are successively shorter.  There is a small opening in the skin at the end of each digit for a claw that is usually reduced to a vestigial nodule, and rarely emerges above the skin.  The claw openings are set back from the free edge of the flippers by cartilaginous rods that extend the length of each digit, and expand the size of the flippers.

The hind flippers have dark, short, sparse hair covering part of the proximal end of the flipper, and the rest of the dorsal surface, and the entire sole is covered in black leathery hairless skin.  The hind flippers are long and each digit has a cartilaginous rod that adds a flap-like extension to each toe.  The bones of the three central toes terminate at the position of the small nails that emerge through the skin on the dorsal surface, set back from the end of the flipper.  The claws of digits 1 and 5 are vestigial like the claws on the fore flippers, and may or may not emerge from small openings set back from the end of the flaps.  All of the flaps at the end of the flipper are of relatively equal length.  The first toe or hallux and the fifth toe are somewhat wider than toes 2–4.   

Coloration of adult males is dark grayish brown to grayish black.  The longer guard hairs of the mane are often light tipped with off-white to buff tones on the crown, nape, and upper neck, creating a grizzled or gingered appearance.  Adult females, subadults, and juveniles are dark brown to grayish-black above and paler in variable patterns below, especially on the chest and underside of the neck, which can be tan to creamy gray.  Both sexes have a variable amount of lighter buff to reddish brown color on the muzzle that often extends into the face and above the eyes.  Pups are born in a black coat that soon lightens to a dark brown, and they probably molt to a juvenile coat several months after birth, but this has not been well-documented.

Adult male vocalizations include a single sharp, almost explosive, puff used in conflicts with other males, a bark that is repetitive, high-pitched with nasal qualities, and is “whickery,” and a guttural growl.  Females growl with their mouth open when angered or threatening another animal, and use a low-pitched, prolonged “bawl” for a pup attraction call.  Young animals have a high-pitched roar; a cough which is less sharp and explosive than the puff of adult males, used when they threaten each other; and a bark used during play.

Adult males may reach 2 m. Two adult males that were measured, were approximately 1.8 and 1.9 m in length.  The 1.9 m male was estimated to weigh 160-170 kg.  Adult females average 1.2 m and reach approximately 1.4 m, and weigh 40-50 kg.  Pups are estimated to be 50–60 cm long.

The dental formula is I 3/2, C 1/1, PC 6/5.

Three other otariids, California and Steller sea lions and the northern fur seal share the range of the Guadalupe fur seal.  Coloration of Guadalupe fur and northern fur seals is similar.  Guadalupe fur seals have fur on the dorsum of the fore flipper beyond the wrist, proportionately shorter rear flippers that only have moderate length cartilaginous extensions, and a long tapering muzzle with a bulbous nose that makes the muzzle seem slightly upturned on the end.  Distinguishing females and young can be difficult if not seen well.

At sea, both species can be seen actively grooming while at the surface.  Guadalupe fur seals will rest in a posture characteristic of fur seals of the genus Arctocephalus with the head down and both rear flippers held in the air and apart forming a “Y” shape.  Northern fur seals do not use this posture and routinely sleep in a “jug handle” position with the palm of one fore flipper draped over the soles of the hind flippers which are rotated forward to meet the fore flipper.  Other fur seal species of the genus Arctocephalus use this posture, so it is possible that Guadalupe fur seals use it as well, although it has not been described for the species.  The head is often held up at approximately a 45º angle while the seal is in this position.  Guadalupe fur seals can be readily separated from both sea lion species, based on differences in pelage density and coloration, length and pointedness of the muzzle, relative size and prominence of the ear pinnae, overall size, and long hand flippers with equal length toes.

In California sea lions, the muzzle tapers to a blunt end, and the first toe or hallux is larger and longer than all of the other toes.  Adult female, subadult, and juvenile California sea lions are tan to pale brown and much lighter in coloration than Guadalupe fur seals. Only adult and subadult male California sea lions become dark brown and are similar in color to Guadalupe fur seal bulls.  However, dark male sea lions have a sagittal crest in contrast to the head of Guadalupe fur seal bulls that is slightly domed and lacks a conspicuous sagittal crest.  California sea lion bulls lack the light hairs in their much shorter mane.  The loud repetitive bark of male California sea lions is distinctive and different from all Guadalupe fur seal vocalizations.  Steller sea lions are very large with a massive head, blunt thick muzzle, stocky body, pale color, and short fur, and should never present an identification problem with Guadalupe fur seals.