Thalassoleon ("sea lion"[1]) is an extinct genus of large fur seal.

Thalassoleon inhabited the Northern Pacific Ocean in latest Miocene and early Pliocene.

T. mexicanus was comparable in size to the largest modern fur seals, with an old-male skull length of 272 mm and an estimated minimal weight of 295–318 kg (650–700 lb).

[1] Holotype of T. macnallyae, UCMP 112809, is a male equal in size to T.

This article related to prehistoric animals from order Carnivora is a stub.