Saint Patrick (Latin: Pātricius; Irish: Pádraig Irish pronunciation: [ˈpˠɑːɾˠɪɟ] or Irish pronunciation: [ˈpˠaːd̪ˠɾˠəɟ]; Welsh: Padrig) was a fifth-century Romano-British Christian missionary and bishop in Ireland. Known as the "Apostle of Ireland", he is the primary patron saint of Ireland, the other patron saints being Brigid of Kildare and Columba. He is also the patron saint of Nigeria. Patrick was never formally canonised by the Catholic Church, having lived before the current laws were established for such matters. He is venerated as a saint in the Catholic Church, the Lutheran Church, the Church of Ireland (part of the Anglican Communion), and in the Eastern Orthodox Church, where he is regarded as equal-to-the-apostles and Enlightener of Ireland.

The dates of Patrick's life cannot be fixed with certainty, but there is general agreement that he was active as a missionary in Ireland during the fifth century. A recent biography on Patrick shows a late fourth-century date for the saint is possible. According to tradition dating from the early Middle Ages, Patrick was the first bishop of Armagh and Primate of Ireland, and is credited with bringing Christianity to Ireland (despite evidence of some earlier Christian presence on the island), and converting Ireland from paganism in the process.

In Patrick's autobiographical Confessio, he writes that when he was about sixteen, he was captured by Irish pirates from his home in Britain and taken as a slave to Ireland. He writes that he lived there for six years as an animal herder before escaping and returning to his family. After becoming a cleric, he returned to spread Christianity in northern and western Ireland. In later life, he served as a bishop, but little is known about where he worked. By the seventh century, he had already come to be revered as the patron saint of Ireland.

Saint Patrick's Day, considered his feast day, is observed on 17 March, the supposed date of his death. It is celebrated in Ireland and among the Irish diaspora as a religious and cultural holiday. In the Catholic Church in Ireland, it is both a solemnity and a holy day of obligation.

Two Latin works survive which are generally accepted as having been written by St. Patrick: the Declaration (Latin: Confessio) and the Letter to the soldiers of Coroticus (Latin: Epistola), from which come the only generally accepted details of his life. The Declaration is the more biographical of the two. In it, Patrick gives a short account of his life and his mission. Most available details of his life are from subsequent hagiographies and annals, which have considerable value but lack the empiricism scholars depend on today.

The only name that Patrick uses for himself in his own writings is Pātricius Latin pronunciation: [paːˈtrɪ.ki.ʊs], which gives Old Irish: Pátraic Irish pronunciation: [ˈpˠaːd̪ˠɾˠəɟ] and Irish: Pádraig (Irish pronunciation: [ˈpˠaːd̪ˠɾˠəɟ] or Irish pronunciation: [ˈpˠɑːɾˠɪɟ]); English Patrick; Scottish Gaelic: Pàdraig; Welsh: Padrig; Cornish: Petroc.

Hagiography records other names he is said to have borne. Tírechán's seventh-century Collectanea gives "Magonus, that is, famous; Succetus, that is, god of war; Patricius, that is, father of the citizens; Cothirthiacus, because he served four houses of druids." "Magonus" appears in the ninth-century Historia Brittonum as Maun, descending from British *Magunos, meaning "servant-lad". "Succetus", which also appears in Muirchú moccu Machtheni's seventh-century Life as Sochet, is identified by Mac Neill as "a word of British origin meaning swineherd". Cothirthiacus also appears as Cothraige in the 8th-century biographical poem known as Fiacc's Hymn and a variety of other spellings elsewhere, and is taken to represent a Primitive Irish: *Qatrikias, although this is disputed. Harvey argues that Cothraige "has the form of a classic Old Irish tribal (and therefore place-) name", noting that Ail Coithrigi is a name for the Rock of Cashel, and the place-names Cothrugu and Catrige are attested in Counties Antrim and Carlow.

Muirchú summarizes that these four names correspond to Patrick's different roles at different periods of this life: "Patrick son of <C>alforni(us) had four names: Sochet when he was born, Cothriche when he was a slave, Mauonius when he studied, Patrick when he was consecrated."