Italian doctor of the church, one of the chief 18th-century moral theologians, and founder of the Redemptorists (Redemptorist), a congregation dedicated primarily to parish and foreign missions. In 1871 he was named a doctor of the church by Pope Pius IX. In 1950 he was named patron saint of moralists and confessors by Pope Pius XII.

After practicing law for eight years, he was ordained a priest in 1726. In 1732 he founded the Congregation of the Most Holy Redeemer, or the Redemptorists, at Scala. Dissension within the congregation culminated in 1777 when he was deceived into signing what he thought was a royal sanction for his rule. Actually, the document was a new rule devised by one of his enemies, thus causing the followers of the old rule to break away. In 1762 Pope Clement XIII made him bishop of Sant' Agata del Goti near Naples; he resigned in 1775 because of ill health.

Liguori's extensive works fall into three genres: moral theology, best represented by his celebrated Theologia moralis (1748); ascetical and devotional writings, including Visits to the Blessed Sacrament, The True Spouse of Jesus Christ (for nuns), Selva (for priests), and The Glories of Mary—one of the most widely used manuals of devotion to the Virgin Mary; and dogmatic writings on such subjects as papal infallibility and the power of prayer. By the middle of the 20th century, his works had gone through several thousand editions and had been translated into 60 languages. In theology Liguori is known as the principal exponent of equiprobabilism, a system of principles designed to guide the conscience of one in doubt whether he is free from or bound by a given civil or religious law.