South American fur seals are sexually dimorphic.  Adult males are approximately 1.3 times longer and 3.3 times heavier than adult females.  Both adult males and females have a stocky robust build, large prominent eyes, long conspicuous ear pinnae, and moderate length yellowish white vibrissae.  The muzzle is moderately long, straight, and tapers in width and thickness to the nose.  The rhinarium is slightly enlarged on the top, contributing to the somewhat upturned look to the end of the muzzle in profile.  The crown is rounded and there is a conspicuous forehead that is steeper in adult males.

Adult males have a very thick neck and massive shoulders.  Males develop a mane of longer guard hairs between the crown and the shoulders, including the neck and upper chest.   The canine teeth of adult males are larger and thicker than those of females.

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. 

Adult females and subadults are dark brown to grayish black above and paler, often with mixed shades of tan, buff, grayish, and rusty brown below.  The chest, and ventral part of the neck, and sides of the neck are light colored to a variable extent.  The head and face are dark, but the sides of the muzzle in the mystacial area are pale.  The ear pinnae are pale colored, and in females and juveniles the fur around where the pinnae attach can also be lighter colored. The fur on the top of the flippers is generally quite dark.  As they age, males darken and become more uniformly-colored, generally dark brown, with gray to yellowish tan grizzled frosting.  Some bulls are paler.  At birth, pups have a longer blackish fur, but there may be some paler markings on the face and muzzle, and some animals are paler below.

Adult males reach 1.9 m and 120–160, and possibly 200 kg, females are about 1.4 m and 40-50 kg.  Newborns are 60-65 cm and 3.5-5.5 kg.  The dental formula is I 3/2, C 1/1, PC 6/5.

South American fur seals share most of their range with South American sea lions.  Five other otariids: Juan Fernandez, Antarctic, Subantarctic, and Galapagos fur seals, and Galapagos sea lions could be found as vagrants within the range of the South American fur seal.

South American sea lions have large, blocky heads with a short muzzle that is blunt on the end, and smallish inconspicuous ear pinnae.  They are also pale tan to light golden brown, with sleek fur that is not dense or shaggy, except in the case of the mane on otherwise unmistakable adult males, and are heavy bodied.

Female, juvenile, and subadult Galapagos sea lions are also pale tan to light golden brown, with sleek fur that is not dense or shaggy.  Adult male Galapagos sea lions are dark brown to black, and thus similar in color to South American fur seals, but have a conspicuous sagittal crest on the crown behind the eyes and lack a conspicuous mane.  All Galapagos sea lions have a proportionately wider muzzle that is blunt on the end, with less conspicuous vibrissae, and proportionately smaller eyes and shorter ear pinnae.

Adult male Juan Fernandez fur seals have a longer muzzle with a bulbous enlarged rhinarium and significantly downward angled nares.  The mane of adult males is grizzled principally on the crown, nape and upper neck whereas adult male South American fur seals are grizzled more evenly throughout the entire mane.

Galapagos fur seals are small and stocky.  The muzzle is very short and tapers rapidly to a sharp point.  The rhinarium is small and unremarkable.  Adult males have a paler “mask” of short lighter colored pelage on the muzzle and face to the eyes.  Because of their small size the head of adult females seems proportionately wide. 

Distinguishing adult male Antarctic fur seals from adult male South American fur seals can be problematic due to similar size and coloration.  Adult male Antarctic fur seals have a sorter muzzle without any enlargement to the rhinarium and no upward angle at the end.  Also, Antarctic fur seals have proportionately longer fore and hind flippers.  Subantarctic fur seals are uniquely colored.  The back and rump of adult males is dark gray to brownish black, and the chest and neck through the muzzle, face and area around the eyes are cream with yellow to orange shading.  The ear pinnae are short, mostly bare skin and dark, and attach in the dark pelage of the upperparts.  There is also a tuft of longer pale fur in the dark crown above the eyes and behind the dark forehead.  Although the adult male is enlarged and thick in the shoulders and neck, he does not have a mane.   Adult females have similar coloration to the males, but have less well-defined pale facial markings that can be shaded with dull yellow orange to light brown.  In both sexes the top of the fore flippers is darker as is the area where the flippers attach to the sides.  The underside of the abdomen is dark ginger to reddish brown.  The fore flippers and hind flippers are proportionately short and wide.

Identifying single adult female, subadult, and juvenile southern fur seal species at a distance, away from conspecifics, is usually difficult and often impossible without extensive experience with the species in question, especially in an area like the southern coasts of South America where vagrants of a number of species are possible.

South American fur seals are widely-distributed from central Peru, around the southern tip of the continent, and north to southern Brazil.  They also occur in the Falkland Islands.  Distribution at sea is poorly known.  These seals are thought to forage primarily in continental shelf and slope waters.  However, there are records from more than 600 km offshore.