A moon of Neptune was discovered by the Voyager Imaging Team in September, 1989. Thalassa is also designated as "Neptune IV" and "S/1989 N5". In Greek mythology, Thalassa was a daughter of Aether and Hemera.

Thalassa is a tiny moon of Neptune. It was discovered using NASA's Voyager 2 mission in 1989. It orbits 50,070 km from the center of Neptune and is about 80 km in diameter.

Thalassa (thÉ™-las'-É™, , Greek Θάλασσα), or Neptune IV, is the second innermost satellite of Neptune. Thalassa was named after a daughter of Aether and Hemera from Greek mythology. "Thalassa" is also the Greek word for "sea".