In ancient Greek mythology and religion, Leto (; Ancient Greek: Λητώ, romanized: Lētṓ pronounced [lɛːtɔ̌ː]) is a goddess and the mother of Apollo, the god of music, and Artemis, the goddess of the hunt. She is the daughter of the Titans Coeus and Phoebe, and the sister of Asteria.

In the Olympian scheme, the king of gods Zeus is the father of her twins, Apollo and Artemis, who Leto conceived after her hidden beauty accidentally caught the eye of Zeus. Classical Greek myths record little about Leto other than her pregnancy and search for a place where she could give birth to Apollo and Artemis, since Hera, the wife of Zeus, in her jealousy ordered all lands to shun her and deny her shelter. Hera is also usually the one to have sent the monstrous Python, a giant serpent, against Leto to pursue and harm her. Leto eventually found an island, Delos, that was not joined to the mainland or attached to the ocean floor, therefore it was not considered land or island and she could give birth. In some stories, Hera further tormented Leto by delaying her labour, leaving Leto in agony for days before she could deliver the twins, especially Apollo. Once Apollo and Artemis are born and grown, Leto withdraws, to remain a matronly figure upon Olympus, her part already played.

Besides the myth of the birth of Artemis and Apollo, Leto appears in other notable myths, usually where she punishes mortals for their hubris against her. After some Lycian peasants prevented her and her infants from drinking from a fountain, Leto transformed them all into frogs inhabiting the fountain. In the story of Niobe, Queen Niobe boasts of being a better mother than Leto due to having given birth to fourteen children, as opposed to only two. Leto asks her twin children to avenge her, and they respond by shooting all of Niobe's sons and daughters dead as punishment. In another myth, the gigantic Tityos attempted to violate Leto, only for him to be slain by Artemis and Apollo. Usually, Leto is found at Olympus among the other gods, having gained her seat next to Zeus, or accompanying and helping her son and daughter in their various endeavors.

In antiquity, Leto was usually worshipped in conjunction with her twin children, particularly in the sacred island of Delos, as a kourotrophic deity, the goddess of motherhood; in Lycia she was a mother goddess. In Roman mythology, Leto's Roman equivalent is Latona, a Latinization of her name, influenced by Etruscan Letun. In ancient art, she is presented as a modest, veiled women in the presence of her children and Zeus, usually in the process of being carried off by Tityos.