Pinnipeds are seals, sea lions, and walruses. Some scientists classify the pinnipeds as a suborder of the order Carnivora.

The odobenids share some characteristics with the other two pinniped families. Morphologically, the walrus is more similar to the Phocidae (true seals). Behaviorally, they more closely resemble the Otariidae (eared seals). Several studies based on molecular data have linked the walrus more closely to the otariids, yet the pinniped family that the odobenids are most closely allied to is still in question.

Some researchers divide the Odobenidae into two subfamilies: the Odobeninae (living walruses of the genus Odobenus and six extinct walrus genera) and the Dusignathinae (four extinct walrus genera).

Most scientists recognize two subspecies of walruses: Odobenus rosmarus rosmarus (Atlantic) and Odobenus rosmarus divergens (Pacific). Odobenus comes from the Greek: "tooth walker", and refers to the walruses' method of pulling themselves up onto the ice with their long tusks.

The common name, walrus, originated with the Danish word hvalros, meaning "sea horse" or "sea cow". The Russian word for walrus is morzh. Arctic natives call the walrus aivik (Inuit) or aivuk (Yu'pik).

The earliest of the odobenid fossils dates back to the middle Miocene, about 14 million years ago.

From the fossil record, scientists believe that the Dusignathinae (fossil walruses) were abundant in the North Pacific 11 to 14 million years ago. Unlike the modern walrus with its elongated upper canines (tusks), upper and lower canine teeth of these walruses were about the same size.

Scientists theorize that the ancestors of the Odobeninae, or modern walrus, probably made their way from the northern Pacific Ocean to the Atlantic during the late Miocene, about 6.5 million years ago, by way of a Central American seaway.

Within the last one million years, walruses probably re-entered the Pacific via the Arctic. The modern Pacific walrus originated from the Atlantic stock.