James of Nisibis (Latin Jacobus Nisibenus; Syriac Mar Ya'qub) was the foundational Christian bishop of the Mesopotamian frontier city of Nisibis (modern Nusaybin in southeastern Turkey, on the Turkish-Syrian border). He was consecrated bishop around 308 AD and governed the see for thirty years until his death around 338. The city of Nisibis was the principal Roman Mesopotamian frontier city — the eastern military-administrative bulwark of the Roman Empire against the Persian Sasanian dynasty.
James founded — within the great cathedral compound at Nisibis — the School of Nisibis, which would become the foundational theological school of Syriac Christianity through the 5th and 6th centuries (and which would migrate to Edessa in the 5th century after the Sasanian conquest of Nisibis). The young Ephrem the Syrian (see Ephrem's entry already in the catalogue) was James's most distinguished personal pupil; Ephrem accompanied James to the Council of Nicaea in 325 as his deacon-assistant.
James was one of the 318 bishops at the Council of Nicaea — one of the relatively few Eastern-Mesopotamian bishops attending, providing the Eastern-Syriac voice in the foundational definition of the Nicene Creed. He returned to Nisibis after the Council and continued to govern the see.
In 337–338 — at the very end of James's life — the Sasanian King Shapur II opened a great military siege of Nisibis (the first of three sieges across the next twenty years). The contemporary tradition (preserved by Theodoret of Cyrus) holds that James — in extreme old age — directed the defence from the cathedral through prayer, and that the city was saved by a miraculous summer plague of mosquitoes that decimated the Persian besieging army. He died shortly after the lifting of the siege. The city of Nisibis honoured him as principal civic-religious patron; the cathedral (the Mor Yakup Church) preserves his cell and tomb to this day, despite the centuries of Islamic rule of the area.