Temporal range: Miocene
Thalassoleon mexicanus by avancna-d1892d6
A restoration of a male (bull; top) and female (cow; bottom) Thalassoleon mexicanus
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Carnivora
Family: Otariidae
Genus: Thalassoleon
1Repenning & Tedford, 1977
Referred species
  • ?Thalassoleon inouei
  • Thalassoleon macnallyae
  • Thalassoleon mexicanus

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

Thalassoleon mexicanus was comparable in size to the largest fur seals, with an estimated weight of 295-318 kg (650-700 lb). Thalassoleon macnallyae, based on size of the mandible, may have grown much larger, similar in size to a walrus.