The Leopard seal Hydrurga leptonyx, also referred to as the sea leopard, is the second biggest species of seal in the Antarctic (after the southern elephant seal). It is most common in the Southern Hemisphere along the coast of Antarctica and on most sub-Antarctic islands, but can also be found on the coasts of southern Australia, Tasmania, South Africa, New Zealand, Lord Howe Island, Tierra del Fuego, the Cook Islands, and the Atlantic coast of South America. It can live 26 years, possibly more.

Leopard seals are predators, feeding mainly on other seals, penguins, fish and krill. Killer Whales are the only known, albeit infrequent, natural predators of leopard seals.

Along with all of the other earless seals, it belongs to the family Phocidae, and is the only species in the genus Hydrurga. The name hydrurga means "water worker" and leptonyx is the Greek for "small clawed".

The leopard seal is large and muscular, with a dark grey back and light grey on its stomach. Its throat is whitish with the black spots that give the seal its common name. The overall length of this seal is 8.4–11.7 ft and weight is from 440 to 1,300 lb. Its front teeth are sharp like those of other carnivores, but its molars lock together in a way that allows them to sieve krill from the water, in the manner of the crabeater seal.

The leopard seal is a solitary creature and comes together in small groups only when it is time to mate. During the mating season, males and females make acoustic calls to each other over distances, with at least the males having individual variability in their vocalising sequence patterns. After a 9-month gestation, the female digs a hole in the ice, and gives birth to a single pup during the Antarctic summer. She protects the pup until it is able to fend for itself.

The leopard seal is bold, powerful and curious. In the water, there is a fine line between curiosity and predatory behavior, and it may 'play' with penguins it does not intend to eat. There are also records of leopard seals attacking divers. Paul Nicklen, a National Geographic magazine photographer, captured pictures of a leopard seal bringing live, injured, and then dead penguins to him, possibly in an attempt to teach the photographer how to hunt.

The leopard seal is second only to the killer whale (orca) among Antarctica's top predators. Its canine teeth are 1 in. It feeds on a wide variety of creatures. Smaller seals probably eat mostly krill, but also squid and fish. Larger leopard seals probably switch from krill to more substantial prey, including king, adelie, rockhopper, gentoo, emperor penguins and chinstraps, and less frequently, other seals, such as crabeater seal.

When hunting penguins, the leopard seal patrols the waters near the edges of the ice, almost completely submerged, waiting for the birds to enter the ocean. It kills the swimming bird by grabbing the feet, then shaking the penguin vigorously and beating its body against the surface of the water repeatedly until the penguin is dead. Previous reports stating the leopard seal skins its prey before feeding have been found to be incorrect. Lacking the teeth necessary to slice its prey into manageable pieces, it flails its prey from side to side to tear and rip it into smaller pieces.

Leopard seals are potentially highly dangerous towards humans, but attacks are rarely reported. Examples of aggressive behavior, stalking and attacks have been documented.

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