A nymph known in several different parts of Greece, usually the Pelopponnese and Sicily. She was one of the Nereids. The river-god Alpheus fell madly in love with her, but she fled to Sicily. There she was changed into a fountain (the Fonte Aretusa, in Syracuse) by Artemis. Apheus made his way beneath the sea, and united his waters with those of Arethusa.

On coins from Syracuse the head of Arethusa was often portrayed (ca. 500 BC). This girls' head has often a net in her hair and is usually surrounded by fish.

Arethusa is also the name of one of the Hesperides.