Killer whales, some species of sharks, Arctic wolves, polar bears, cougars, brown hyenas, various canid species and even other types of seals prey on seals. Some species of seals don't have any natural predators due to where they live or their size.
Killer whales, or orcas as they're also called, are the most prominent threat to seals. Killer whales use their powerful tails to flip seals in the air and slap them, and they also use their heads to ram into seals. Seals are most commonly hunted by groups of up to 10 killer whales, although some travel and hunt in larger groups, or alone. Young seals are the primary target for killer whales, although adults are hunted as well.
Large sharks, like tiger sharks, mako sharks and especially the great white shark, are also major predators of seals. Sharks attack seals by sneaking up on them from below, and many seals survive with wounds. Polar bears actively stalk seals on ice or on water, and sometimes they prefer to sit and wait until unsuspecting prey come near. Terrestrial hunters such as cougars, hyenas and various canids, tend to hunt seal pups because they're easier to target than adult seals. Leopard seals have been known to hunt and eat various other species of seals.