A marine mammal is a mammal that is primarily ocean-dwelling or depends on the ocean for its food. There are five groups of marine mammals:

1. Order Sirenia: the manatee, dugong, and sea cow
2. Order Carnivora, family Ursidae: the polar bear
3. Order Carnivora, infrafamily Pinnipedia: the seal, sea lion, and walrus
4. Order Carnivora, family Mustelidae: the otter
5. Order Cetacea: the whale, dolphin, and porpoise

Mammals evolved on land, and their spines are optimized for running on all four legs, allowing for up and down but only little sideways motion. Marine mammals therefore typically swim by moving their spine up and down, while fish normally swim by moving their spine sideways.

All mammals have hair. The marine mammals, with the exception of polar bears and otters, have lost most of their hair, thus decreasing water resistance. They rely on insulation by a layer of fat instead. Since the different groups of marine mammals originate from different ancestors, this is a case of convergent evolution.

Note that the polar bear spends a large proportion of its time in a marine environment, albeit a frozen one. When it does swim in the open sea; it is extremely proficient and can cover up to 60km in a day. For these reasons, some scientists regard it as a marine mammal.