Photios I (c. 810 – c. 893) also spelled Photius or Fotios, is the Ecumenical Patriarch of Constantinople from 858 to 867 and from 877 to 886.

He is recognized in the Eastern Orthodox churches as St. Photios the Great.

Photios is widely regarded as the most powerful and influential Patriarch of Constantinople since John Chrysostom, and as the most important intellectual of his time.

He is a central figure in both the conversion of the Slavs to Christianity and the Photian schism.

Photios is a well-educated man from a noble Constantinopolitan family.

Photius's great uncle was the previous Patriarch of Constantinople, Tarasius.

He intended to be a monk, but chose to be a scholar and statesman instead.

In 858, Emperor Michael III (r. 842–867) deposes Patriarch Ignatius of Constantinople, and Photios, still a layman, is appointed in his place.

Amid power struggles between the pope and the Byzantine emperor, Ignatius is reinstated.

Photios resumes the position when Ignatius dies (877), by order of the Byzantine emperor.

The new pope, John VIII, approves Photios's reinstatement.

Catholics regard a Fourth Council of Constantinople (Roman Catholic) as anathematizing Photios as legitimate.

Eastern Orthodox regard a second council named the Fourth Council of Constantinople (Eastern Orthodox), reversing the first, as legitimate.

The contested Ecumenical Councils mark the end of unity represented by the first seven Ecumenical Councils.