Pope Saint Nicholas I (Latin: Nicholaus I; c. 800 – 13 November 867), also called Saint Nicholas the Great, is the head of the Catholic Church from 24 April 858 to his death in 867.

He is remembered as a consolidator of papal authority and power, exerting decisive influence upon the historical development of the papacy and its position among the Christian nations of Western Europe.

Nicholas assertes that the pope should have suzerain authority over all Christians, even royalty, in matters of faith and morals.

e is venerated as a saint in the Roman Catholic Church, with a feast day on November 13.

He refuses to grant an annulment to King Lothair II of Lotharingia from Teutberga so that Lothair can marry his mistress Waldrada.

When a Council pronounces in favor of annulment, Nicholas I declares the Council to be deposed, its messengers excommunicated, and its decisions void.

Despite pressure from the Carolingians, who lay siege to Rome, his decision holds.

During his reign, relations with the Byzantine Empire sour over his support for Ignatius, Patriarch of Constantinople, who had been removed from his post in favor of Photius.