Euthymius I of Constantinople was a 9th-century Byzantine theologian and priest who served as the Ecumenical Patriarch from 907 to 912. Born in Silifke, he entered monastic life at a young age and eventually became the spiritual mentor to the future emperor Leo VI the Wise. Although he held the high office of syncellus, his relationship with the emperor was often strained due to his strict adherence to canon law, particularly regarding the imperial marriages of Leo VI.