Bishop of Rome, succeeded Julius I. in 352, during the dominancy of the semi-Arians. Soon after his accession, it is said, he excommunicated Athanasius, the great antagonist of Arianism; but when the councils of Arles and Milan, overawed by the Emperor Constantius, deposed that bishop in 355, Liberius, changing his policy, could not be induced, either by compulsion or blandishments, to sign the deposition. Hurried in secrecy from Rome during the night, and carried before the Emperor in Milan, he still remained faithful to Athanasius, and was banished to Bercea, in Macedonia. After an exile of two years, however, he withdrew his protest, and signed the semi-Arian creed of Sirmium. The Emperor, overcome by the pertinacious petitions of the ladies at Rome, then permitted him to share the management of his see with Felix, who had succeeded during his banishment. The latter was forced to flee from popular violence, and Liberius, left in the peaceful possession of his dignity, expiated, it is said, his defection from orthodoxy, by a sincere recantation. He died in 366.

Liberius is the reputed author of twelve epistles, published in Constant's Epistolae Pontificum Romanorum, Paris, 1721; and he is said to have built the Basilica, on the Esquiline Mount, which from him has sometimes been called Liberiana, but is now known as the Santa Maria Maggiore.