Dionysius I of Constantinople was a 15th-century prelate who served as the Ecumenical Patriarch of Constantinople during two separate periods. Born in Dimitsana, he became a monk and a priest under the guidance of Mark of Ephesus. After being enslaved during the fall of the city, he was eventually freed and rose to prominence through the support of Mara Branković. His leadership was marked by intense political factionalism within the church and the Ottoman administration.