In Greek mythology, Leucippus was a maiden of Phaestus, Crete, who was raised as a boy by her mother, and eventually changed her sex to male by the will of the goddess Leto.
Leucippus was born to Lamprus, the son of Pandion, and Galatea, daughter of Eurytius the son of Sparton. When Galatea was pregnant, Lamprus told her he would only accept the child if it was male, but Galatea gave birth to a daughter while Lamprus was away pasturing his cattle. Following the advice of seers, Galatea gave her daughter a masculine name, Leucippus, and told Lamprus that she had given birth to a son. Leucippus was raised as a boy, but as she reached the age of adolescence, it was becoming difficult to conceal her true sex, so Galatea went to the sanctuary of Leto and prayed that her daughter may be changed into a son, invoking a series of mythical examples of magical sex change, such as the story of Caeneus, Teiresias, Hypermnestra (otherwise known as Mnestra) who sold herself as a woman and then changed into a man to return home, and Sypretes of Crete who transformed into a woman to see Artemis bathing. Leto took pity on Galatea and changed Leucippus' sex to male. In commemoration of this event, people of Phaestus surnamed Leto Phytia (from Greek φύω "to grow"), because she let Leucippus grow a penis, and established a feast in honor of Leto, which was called Ecdysia (from Greek ἑκδύω "to undress"), because Leucippus got rid of woman's clothes after his transformation. It was a custom for women of Phaestus to lie next to the statue of Leucippus before their wedding.
The story of Leucippus is similar to that of Iphis the daughter of Ligdus and Telethusa.