Ustad Sis or Master of Sis (Sistan) was a Zoroastrian rebel from Khorasan. Most of his followers came from Behafarid’s movement.

Ustad Sis captured Herat (in today's Afghanistan) and Sistan. But eventually Caliph al Mahdi, whose Dirham is shown on the right, defeated him and captured his 70,000 followers. Ustad Sis was later executed in Baghdad. Some of his followers, however, were later pardoned by the Caliph.