Euthymius I Syncellus (Greek: Εὐθύμιος Α΄ ὁ Σύγκελλος, c. 834 – 5 August 917) was the Ecumenical Patriarch of Constantinople from 907 to 912. A monk since his youth, he became spiritual father of the future emperor Leo VI the Wise, and was raised by him to the high ecclesiastical office of syncellus. Despite his turbulent relationship with Leo, in 907 he was appointed to the patriarchate and held the post until his deposition shortly before or after Leo's death in 912.