The crabeater seal (Lobodon carcinophaga), also known as the krill-eater seal, is a true seal with a circumpolar distribution around the coast of Antarctica. They are the only member of the genus Lobodon. They are medium- to large-sized (over 2 m (6.6 ft) in length), relatively slender and pale-colored, found primarily on the free-floating pack ice that extends seasonally out from the Antarctic coast, which they use as a platform for resting, mating, social aggregation and accessing their prey. They are by far the most abundant seal species in the world. While population estimates are uncertain, there are at least 7 million and possibly as many as 75 million individuals. This success of this species is due to its specialized predation on the abundant Antarctic krill of the Southern Ocean, for which it has uniquely adapted, sieve-like tooth structure. Indeed, its scientific name, translated as "lobe-toothed (lobodon) crab eater (carcinophaga)", refers specifically to the finely lobed teeth adapted to filtering their small crustacean prey.

Despite its name, the crabeater seal does not eat crabs.

As well as being an important krill predator, the crabeater seal's pups are an important component of the diet of leopard seals (H. leptonyx).

The genus name of the crabeater seal, Lobodon, derives from Ancient Greek meaning "lobe-toothed", and the species name carcinophaga means "crab eater". The crabeater seal shares a common recent ancestor with the other Antarctic seals, which are together known as the lobodontine seals. These include the leopard seal (Hydrurga leptonyx), the Ross seal (Ommatophoca rossii), and the Weddell seal (Leptonychotes weddelli). These species, collectively belonging to the Lobodontini tribe of seals, share teeth adaptations including lobes and cusps useful for straining smaller prey items out of the water column. The ancestral Lobodontini likely diverged from their sister clade, the Mirounga (elephant seals) in the late Miocene to early Pliocene, when they migrated southward and diversified rapidly in the relative isolation around Antarctica.

Adult seals (over five years old) grow to an average length of 2.3 m (7.5 ft) and an average weight of around 200 kg (440 lb). Females are on average 6 cm (2.4 in) longer and around 8 kilograms (18 lb) heavier than males, though their weights fluctuate substantially according to season; females can lose up to 50% of their body weight during lactation, and males lose a significant proportion of weight as they attend to their mating partners and fight off rivals. During summer, males typically weigh 200 kilograms (440 lb), and females 215 kilograms (474 lb). A molecular genetic based technique has been established to confirm the sex of individuals in the laboratory. Large crabeater seals can weigh up to 300 kg (660 lb). Pups are about 1.2 metres (3.9 ft) in length and 20 to 30 kilograms (44 to 66 lb) at birth. While nursing, pups grow at a rate of about 4.2 kilograms (9.3 lb) a day, and grow to be around 100 kilograms (220 lb) when they are weaned at two or three weeks.

These seals are covered mostly by brown or silver fur, with darker coloration around flippers. The color fades throughout the year, and recently molted seals appear darker than the silvery-white crabeater seals that are about to molt. Their body is comparatively more slender than other seals, and the snout is pointed. Crabeater seals can raise their heads and arch their backs while on ice, and they are able to move quickly if not subject to overheating. Crabeater seals exhibit scarring either from leopard seal attacks around the flippers or, for males, during the breeding season while fighting for mates around the throat and jaw. Pups are born with a light brown, downy pelage (lanugo), until the first molt at weaning. Younger animals are marked by net-like, chocolate brown markings and flecks on the shoulders, sides and flanks, shading into the predominantly dark hind and fore flippers and head, often due to scarring from leopard seals. After molting, their fur is a darker brown fading to blonde on their bellies. The fur lightens throughout the year, becoming completely blonde in summer.^{[citation needed]}

Crabeaters have relatively slender bodies and long skulls and snouts compared to other phocids. Perhaps their most distinctive adaptation is the unique dentition that enables this species to sieve Antarctic krill. The postcanine teeth are finely divided with multiple cusps. Together with the tight fit of the upper and lower jaw, a bony protuberance near the back of the mouth completes a near-perfect sieve within which krill are trapped.

Crabeater seals have a continuous circumpolar distribution surrounding Antarctica, with only occasional sightings or strandings in the extreme southern coasts of Argentina, South Africa, Australia and New Zealand. They spend the entire year on the pack ice zone as it advances and retreats seasonally, primarily staying within the continental shelf area in waters less than 600 m (2,000 ft) deep. They colonized Antarctica during the late Miocene or early Pliocene (15–25 million years ago), at a time when the region was much warmer than today. The population is connected and fairly well mixed (panmictic), and genetic evidence does not suggest any subspecies separations. A genetic survey did not detect evidence of a recent, sustained genetic bottleneck in this species, which suggests that populations do not appear to have suffered a substantial and sustained decline in the recent past.