Galápagos Sea Lions are similar in appearance to California Sea Lions, but differ in size, behavior, and skull morphology. Galápagos Sea Lions are sexually dimorphic, with males growing larger than females and having a suite of 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 suggestion.

There is little information on the lengths and weights attained by Galápagos 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. Adult females caring for pups weigh between 50 and 100 kg. Pups of both sexes are born at approximately six kilograms and weaned at approximately 25-40 kg. Pups are born with a brownish-black lanugo coat that fades to pale brown by three to five months. Pups go through their first moult at around five months of age and emerge with the pelage of adult females and juveniles.

Age of maturity for both sexes is estimated to be about 4-5 years. Females produce one pup each year after a gestation of about 11 months, but may abort (or not produce a pup) while still caring for an older offspring (Trillmich and Wolf 2008). Longevity is estimated to be around 15-24 years but the higher ages are not confirmed (Reijnders et al. 1993).

Galápagos Sea Lions are polygynous and males hold territories both on land and in shallow water near shore that they vociferously and aggressively defend. Male tenure on territories usually lasts from ten days to three months. Males may repeatedly be on territory during the drawn out reproductive period. Most copulations occur in the water.

Pupping and breeding take place across an extended period from May through January. Because of the protracted breeding season and extended care provided to the pups (up to 3 years) by females, there are dependent pups on the rookeries year-round. Females may wean pups in 11-12 months in productive years, but most continue to suckle yearlings for a second year. Some females care for a yearling along with a newborn pup. Pups are attended continuously for the first 4-7 days after birth, after which the female goes to sea to feed. With the departure, the female begins a cycle of foraging trips that last 0.5-3 days during the cold season but may last much longer in the warm season. Pups will enter the water and begin to develop swimming skills 1-2 weeks after birth. In some colonies, females return at night to nurse their pup, departing again the next morning (Trillmich 1986). Females and pups recognize each other and reunite based on calls and scent (Trillmich 1981). Galápagos Sea Lion females feed during day and night, in contrast to Galápagos Fur Seals, which primarily feed at night.

Galápagos Sea Lions are non-migratory. They are unafraid of humans when ashore. Haul-out sites can be on rugged shoreline types, including steep rocky shorelines, ledges and offshore stacks, but colonies are mostly on gently sloping sandy and rocky beaches. Sea lions will use shade from vegetation, rocks, and cliffs, and wade into tidal and drainage pools or move into the ocean, as needed during the heat of the day to avoid overheating.

Diving has been studied in four females on Fernandina and about 20 in the centre of the archipelago. The maximum depth of dive recorded was 338 m and maximum duration was 9.8 minutes. Average depth of dives varied between 45 and 150 m and lasted for 3-5 minutes (Villegas-Amtmann et al. in press). At sea they will raft at the surface and rest on their sides with one or more flippers held vertically in the air.

Galápagos Sea Lions prey on sardines in the west and on sardines as well as myctophids and bathylagids together with small squid in the central parts of the archipelago. Galápagos Sea Lions have been seen smashing octopus on the surface of the water, presumably to stun or break them up to facilitate swallowing. During El Niño events even in the west prey includes green-eyes and myctophids, suggesting a change in foraging strategy.

Feral and uncontrolled dogs have been reported to kill sea lion pups. Shark predation is evident from animals seen with injuries and scars from attacks, and killer whales are presumed to be another predator on Galápagos Sea Lions. Interestingly, juvenile and adult Galápagos Sea Lions have been observed to mob Galápagos sharks that approach rookeries.