Yahya ibn Ismail al-Mamun () (died 1075) was the second ruler of the Berber Hawwara Dhulnunid dynasty who was king of the Taifa of Toledo between 1043 and 1075.
Yahya ibn Ismail succeeded his father Ismaïl ibn Dhi 'l-Nun in 1043. He died at Córdoba in 1075.