In Greek mythology, Priapus (/praɪˈeɪpəs/; Ancient Greek: Πρίαπος, romanized: Príapos) is a minor rustic fertility god, protector of livestock, fruit plants, gardens, and male genitalia. Priapus is marked by his oversized penis, and his permanent erection, which gave rise to the medical term priapism. He became a popular figure in Roman erotic art and Latin literature, and is the subject of the often humorously obscene collection of verse called the Priapeia.

Priapus was described in varying sources as the son of Aphrodite by Dionysus; as the son of Dionysus and Chione; as perhaps the father or son of Hermes; or as the son of Zeus or Pan. According to legend, Hera cursed him with inconvenient impotence (he could not sustain an erection when the time came for sexual intercourse), ugliness and foul-mindedness while he was still in Aphrodite's womb, in revenge for the hero Paris having the temerity to judge Aphrodite more beautiful than Hera. In another account, Hera's anger and curse were because the baby had been fathered by her husband Zeus. The other gods refused to allow him to live on Mount Olympus and threw him down to Earth, leaving him on a hillside. He was eventually found by shepherds and was brought up by them.

Priapus joined Pan and the satyrs as a spirit of fertility and growth, though he was perennially frustrated by his impotence. In a ribald anecdote told by Ovid, he attempted to rape the goddess Hestia but was thwarted by an ass, whose braying caused him to lose his erection at the critical moment and woke Hestia. The episode gave him a lasting hatred of asses and a willingness to see them killed in his honour. The emblem of his lustful nature was his permanent erection and his large penis. Another myth states that he pursued the nymph Lotis until the gods took pity on her and turned her into a lotus plant.

"Priapos" is a title given to Eros Phanes in the Orphic Hymn to Protogonos, the "firstborn" god of the Greeks who came from the Cosmic Egg. In The Orphic Hymn to Dionysus, Dionysus is given epithets similar to Protogonos and was thought of as the incarnation of Protogonos, so he was considered both the father of fertility god Priapus and also the incarnation of the primordial Priapus.

As well as the collection known as the Priapeia mentioned above, Priapus was a frequent figure in Latin erotic or mythological verse.

In Ovid's Fasti, the nymph Lotis fell into a drunken slumber at a feast, and Priapus seized this opportunity to advance upon her. With stealth he approached, and just before he could embrace her, Silenus's donkey alerted the party with "raucous braying". Lotis awoke and pushed Priapus away, but her only true escape was to be transformed into the lotus tree. To punish the donkey for spoiling his opportunity, Priapus bludgeoned it to death with his gargantuan phallus. When the same story is recounted later in the same book, Lotis is replaced with the virginal goddess Hestia, who avoids being changed into a tree as the other Olympians come to her rescue. Ovid's anecdote served to explain why donkeys were sacrificed to Priapus in the city of Lampsacus on the Hellespont, where he was worshipped among the offspring of Hermes.

Once, a donkey that had been given human speech by Dionysus challenged Priapus to a contest about which between them had the better penis. Priapus won the contest, and then killed the donkey, which was put by Dionysus among the stars.

The first extant mention of Priapus is in the eponymous comedy Priapus, written in the 4th century BC by Xenarchus. Originally worshipped by Greek colonists in Lampsacus in Asia Minor, the cult of Priapus spread to mainland Greece and eventually to Italy during the 3rd century BC. Lucian (De saltatione) tells that in Bithynia Priapus was accounted as a warlike god, a rustic tutor to the infant Ares, "who taught him dancing first and war only afterwards," Karl Kerenyi observed. Arnobius is aware of the importance accorded Priapus in this region near the Hellespont. Also, Pausanias notes: