(Syriac: ܡܪܝ ܐܝܣܚܩ ܕܢܝܢܘܐ‎; Arabic: إسحاق النينوي; Greek: Ἰσαὰκ ὁ Σύρος), also remembered as Saint Isaac the Syrian, Abba Isaac, Isaac Syrus and Isaac of Qatar, was a 7th-century Church of the East Syriac Christian bishop and theologian best remembered for his written works on Christian asceticism.

He is regarded as a saint in the Assyrian Church of the East, the Chaldean Catholic, and Eastern Orthodox tradition. His feast day falls, together with 4th-century theologian and hymnographer St. Ephrem the Syrian, on January 28.