Ustadh Sis (also spelled: Ustad Sis, or Ostad Sis, Persian: استاد سیس) was a Persian heresiarch and anti-Arab rebel leader.They write that he was once a governor of Khorasan and Alleged father to Ma'mūn’s Iranian mother, Marjil, which makes him Ma'mūn's grandfather. He claimed he was a prophet of God in the eastern fringe of Khorasan in the mid 8th century and managed to gain followers among the villagers in that area. Many were previously followers of Bihafarid, whom the Abbasid commander, Abu Muslim, crushed militarily.