Euthymius I Syncellus is the Ecumenical Patriarch of Constantinople from 907 to 912.

Formerly a syncellus, he comes to the office after Emperor Leo VI forces Nicholas I Mystikos to resign over Nicholas' refusal to sanction the Emperor's fourth marriage.

The Vita Euthymii, or The Life of Euthymius, along with the correspondence of patriarch Nicholas and the writings of Arethas of Caesarea, is one of the main historical sources for the period.