Thalassoleon is an extinct genus of large fur seal. Thalassoleon inhabited the Northern Pacific ocean in latest Miocene and early Pliocene time. Fossils of T. mexicanus are known from Baja California and southern California. T. macnallyae is known from central California, and T. inouei (which may be a synonym of T. macnallyae) is known from Japan. Thalassoleon could be the ancestor of the modern northern fur seal.