Thalassocnus is an extinct genus of semi-aquatic or aquatic marine sloth from the Miocene and Pliocene of South America. Fossils found to date have been from the coast of Peru. They were apparently grazers of sea grass and seaweed. Over time, they apparently shifted from a preference for feeding in shallow water to a preference for deeper waters. They may have used their powerful claws to anchor themselves to the sea floor to facilitate feeding, similar to the behavior of the marine iguana.