In ancient Roman religion and mythology, Liber ("the free one"), also known as Liber Pater ("the free Father") was a god of viticulture and wine, fertility and freedom. He was a patron deity of Rome's plebeians and was part of their Aventine Triad. His festival of Liberalia (March 17) became associated with free speech and the rights attached to coming of age. His cult and functions were increasingly associated with Bacchus and his Greek equivalent Dionysus, whose mythologies he came to share.

Before his official adoption as a Roman deity, Liber was companion to two different goddesses in two separate, archaic Italian fertility cults; Ceres, an agricultural and fertility goddess of Rome's Hellenised neighbours, and Libera, who was either Liber's female equivalent or became so through assimilation. In ancient Lavinium, he was a phallic deity. Latin liber means "free", or the "free one": when coupled with "pater", it means "The Free Father", who personifies freedom and champions its attendant rights, as opposed to dependent servitude. Roman writers of the late Republic and early Empire offer various etymological and poetic speculations based on this trope, to explain certain features of Liber's cult.