Syrian bishop, theologian, and monk whose writings on mysticism became a fundamental source for both Eastern and Western Christians.

Born in Qatar, Isaac became a monk of Bet-Qatraje in Kurdistan, northern Iraq, and was consecrated bishop of Nineveh, near modern Mosul, Iraq, c. 670 by the head of the East Syrian Nestorian Church, an independent Christian body that adhered to a doctrine accentuating Christ's human personhood while minimizing his divinity. After five months he resigned his office. Although the evidence is uncertain, it has been suggested that he resigned because of doctrinal differences with the Nestorians and a closer approximation to the Christology of Greek Orthodoxy (belief affirming in Christ human and divine natures in a single personhood).

Retiring to a solitary monastic life in the desert of Rabban Shapur, Isaac devoted himself to writing on mystical themes. He is reputed to have lost his sight because of assiduous reading. The celebrated 14th-century Syrian theologian Abhdisho bar Berikha records that Isaac wrote seven volumes of ascetical treatises, verse, dialogues, and other writings, of which only 82 chapters on Christian mysticism survive.

Translated at an early date into Arabic, Ethiopic, and Greek, Isaac's extensive writings affected Byzantine and Russian theologians and philosophers from the 10th to the 19th centuries. Latin and, later, Italian and Spanish versions influenced Christian mysticism in the West. The Greek and Latin versions of Isaac's extant works are contained in the series Patrologia Graeca, edited by J.-P. Migne, vol. 86 (1866). An English version, Mystic Treatises, edited and translated by A.J. Wensinck, appeared in 1923, translated from the Syriac.