Galapagos sea lions are similar in appearance to California sea lions, but differ in size, behavior, and skull morphology.  Galapagos sea lions are sexually dimorphic, with males growing larger than females and having several secondary sexual characteristics.  The degree of sexual dimorphism appears to be less than in California sea lions, although few weights and measurements are available for adults to confirm this observation.

Adult males are robust in the neck, chest, and shoulders and are proportionately much smaller in the abdomen.  As males sexually mature the enlarging sagittal crest becomes evident as a bump on the crown.  The crest grows until the male reaches physical maturity, at which time it forms a prominent ridge behind the eyes and creates a steep forehead. Galapagos sea lion males are said to lack the pale pelage coloration on top of the sagittal crest common in California sea lions, although this feature appears to be present in one published photograph.  Also, the skull of adult male Galapagos sea lions has a 20-25% smaller sagittal crest, a shorter muzzle, is about 10% shorter overall and is narrower than the skull of male California sea lions.  As in all otariids the canine teeth of adult males are larger and thicker than those of females.

Adult and subadult male Galapagos sea lions bark in often long repeated sequences.  The bark is loud, rapidly repeated, and distinctive. Females and juveniles do not produce the repetitive bark.  Juveniles, subadults and adults of both sexes will also growl.

Adult females and juveniles do no have a sagittal crest.  When viewed in profile, juveniles have a nearly flat head with little or no forehead.  Adult females have a slight forehead formed by a gentle slope from the crown to the muzzle.  In contrast to adult males, adult females have a long relatively thin neck and a wide torso.

Both sexes have a long and somewhat narrow muzzle that tapers to a slightly pointed nose.  In profile, the face of younger animals is dog-like. The fore flippers have a sparse short fur that extends beyond the wrist onto the middle of the dorsal surface of the flipper in a “V” pattern and 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 posterioraly, giving the flipper a swept back look.  Digits 2-4 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 also have cartilaginous rods that extend the length of 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 first and fifth toes are longer than the three middle toes, and the first toe, or hallux, is longer and wider than the fifth toe.  The hind flippers have short 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. 

Color of Galapagos sea lions is highly variable.  When dry, the coat of adult males ranges from grayish and golden brown to the common dark brown, and most bulls appear blackish or very dark when wet.  Darkening begins when males are subadults and is generally complete when a bull reaches physical maturity.  Adult males can have light gray coloration on their backs.  Adult females, juveniles and young subadult males are pale colored above, and can be many shades of tan to light brown.  There are often light colored areas on the muzzle, and around and above the eyes in both sexes.  The sparse short fur covering a portion of the tops of the flippers can be the same color or darker than the color of the body. Pups are born with a longer brownish-black lanugo coat that fades to pale brown by three to five months.  Pups go through their first molt at around five months and emerge with the pelage of adult females and juveniles.

There is little information on the lengths and weighs attained by Galapagos sea lions, but they are said to be somewhat smaller than California sea lions.  Adult males are estimated to weigh up to 250 kg, but this has not been confirmed through direct study.  Four adult females caring for pups weighed from 50 to 100 kg.  Pups of both sexes are born at approximately six kilograms and weaned at approximately 25 kg.

The dental formula is I 3/2, C 1/1, PC 6/5 (~75%), PC 5/5 (~25%).

Galapagos sea lions share the Archipelago with Galapagos fur seals.  As vagrants they may show up in the range of South American sea lions and South American fur seals, and there is a record of the former from the Galapagos.

Galapagos and South American fur seals have thick pelage and look shaggier, especially when wet, than Galapagos sea lions.  Both fur seals are dark gray to brown and darker than similar-sized sea lions, with the exception of a fully-grown male Galapagos sea lion.  Both fur seals have a more pointed muzzle and proportionately larger eyes and longer ears that stand out farther from the head when they are in the water (or otherwise wet).  Adults of both fur seals have long pale conspicuous vibrissae.  Galapagos fur seals are the smallest otariids, only reaching 1.5 and 1.2 meters for males and females respectively, and are stockier with a shorter neck and body than Galapagos sea lions.

Galapagos sea lions are found throughout the Archipelago on all the major islands and on many smaller islands and rocks.  A colony was established in 1986 at Isla de la Plata, just offshore of mainland Ecuador, and vagrants can be seen from the Ecuadorian coast north to Isla Gorgona in Columbia.  There is also a record from Isla del Coco approximately 500 km southwest of Costa Rica.