Rome 208 Neptune Statue, Trevi Fountain

Oceanus ( /oʊˈsiːənəs/); Ancient Greek: Ὠκεανός, Ōkeanós, pronounced [ɔːkeanós]) was a pseudo-geographical feature in classical antiquity, believed by the ancient Greeks and Romans to be the World Ocean, an enormous river encircling the world.

Strictly speaking, Oceanus was the ocean-stream at the Equator in which floated the habitable hemisphere (οἰκουμένη, oikoumene). In Greek mythology, this world-ocean was personified as a Titan, a son of Uranus and Gaia. In Hellenistic and Roman mosaics, this Titan was often depicted as having the upper body of a muscular man with a long beard and horns (often represented as the claws of a crab) and the lower body of a serpent (cf. Typhon). On a fragmentary archaic vessel of circa 580 BC (British Museum 1971.11-1.1), among the gods arriving at the wedding of Peleus and the sea-nymph Thetis, is a fish-tailed Oceanus, with a fish in one hand and a serpent in the other, gifts of bounty and prophecy. In Roman mosaics, such as that from Bardo (illustration below) he might carry a steering-oar and cradle a ship.