Pope Leo I, also known as Leo the Great (c. 391 or 400 – 10 November 461) is the Bishop of Rome—the Pope—of the Catholic Church from 29 September 440 to his death on 10 November 461.

He is an Italian aristocrat, and is the first pope of the Catholic Church to have been called "the Great".

He is perhaps best known for having met Attila the Hun in 452 and persuading him to turn back from his invasion of Italy.

He is also a Doctor of the Church, most remembered theologically for issuing the Tome of Leo, a document which is foundational to the debates of the Ecumenical Council of Chalcedon.

The Council of Chalcedon, the fourth ecumenical council, deals primarily with Christology, and elucidates the orthodox definition of Christ's being as the hypostatic union of two natures—divine and human—united in one person, "with neither confusion nor division".

The Council of Chalcedon gives rise to the first major schism in Christian history, the Monophysite schism.