Thalassocnus is an extinct genus of apparently semiaquatic (for the geologically oldest species) or fully aquatic (for the geologically most recent species) marine sloth from the Miocene and Pliocene of South America. Fossils found to date have been from the coast of Peru and Chile. They were apparently grazers of sea grass and seaweed. The various species of this genus provides the best-documented case of gradual adaptation to a secondarily aquatic lifestyle.