Thalassa, Protogenos of the sea or its surface, and a personification of the Mediterranean, is described as a daughter of Aether and Hemera, closely identified with the sea goddesses Amphitrite and Tethys, and is also known as Thalatta or Thalath. The Telchines, a family, a class of people, or a tribe, are said to have descended from Thalassa or Poseidon. It is probably owing to this story about their origin, that Eustathius describes them as marine beings without feet, the place of the hands being occupied by fins, though in the same page he also states that originally they were the dogs of Actaeon, who were changed into men.

Coupling with her male counterpart Pontos, she spawned the tribes of fish. Like the other Protogenoi, Thalassa was scarcely personified, instead her form was elemental, the body of the sea itself. In the fables of Aesop, Thalassa appears as a woman formed of sea water, rising up from her native element. Poseidon and Amphitrite were the anthropomorphic gods equivalent to Pontos and Thalassa. In late classical times, the two were also confounded with Okeanos and Tethys.

Thalassa was depicted in Roman-era mosaics as a woman half submerged in the sea, with crab-claw horns, clothed in bands of seaweed, and holding a ship's oar. Yet, a few innominate, heavily weathered and broken pieces of statuary from pre-classical Greece, which nonetheless bare a hint of beauty, according to localised lore, are said to represent the elusive deity.