Arethusa (Ἀρέθουσα) means "the waterer". In Greek mythology, she was a Naiad nymph of the sacred spring which bore her name in the Greek colony of Syrakousa (Syracuse) on the island of Ortygia near Sicily.
She was originally a Nereid who escaped to the fled to the island to escape the amorous pursuit of the river-god Alpheios. There she was transformed into the spring of the same name. Alpheios followed in her wake, flowing beneath the sea to spring forth anew on the Sicilian mainland and mingle his waters with hers.
An engraving by Bernard Picart depicting Alpheus in his attempt to capture Arethusa. The myth of her transformation begins when she came across a clear stream and began bathing, not knowing it was the river god Alpheus. He fell in love during their encounter, but she fled after discovering his presence and intentions, as she wished to remain a chaste attendant of Artemis. After a long chase, she prayed to her goddess to ask for protection. Artemis hid her in a cloud, but Alpheus was persistent. She began to perspire profusely from fear, and soon transformed into a stream. Artemis then broke the ground allowing Arethusa another attempt to flee. Her stream traveled under the earth to the island of Ortygia, but Alpheus flowed through the sea to reach her and mingle with her waters.
During Demeter's search for her daughter Persephone, Arethusa entreated Demeter to discontinue her punishment of Sicily for her daughter's disappearance. She told the goddess that while traveling in her stream below the earth, she saw her daughter looking sad as the queen of Hades.
Arethusa occasionally appeared on coins as a young girl with a net in her hair and dolphins around her head. These coins were common around Ortygia, the location in which she ends up after fleeing from Alpheus.
The Roman writer Ovid called Arethusa by the name "Alpheias", because her stream was believed to have a subterranean communication with the river Alpheius, in Peloponnesus.