Jacob of Nisibis was a 4th-century archbishop and hermit who lived in Mesopotamia. Born in the 3rd century, he served as a spiritual mentor to the theologian Ephrem the Syrian and was a participant in the first ecumenical council at Nicaea. Known as a wonderworker, he is celebrated for his ascetic life in the mountains and his role in defending his city during sieges. He is honored as a saint across several Christian traditions, including the Catholic, Eastern Orthodox, and Oriental Orthodox churches.