The siege of Maymun-Diz, an unlocated fortress and the stronghold of the leader of the Nizari Ismaili state, Imam Rukn al-Din Khurshah, occurred in 1256, during the Mongol campaign against the Nizaris led by Hülegü. The new Nizari Imam was already engaged in negotiations with Hülegü as he was advancing toward his stronghold. The Mongols insisted that all Nizari fortresses be dismantled, but the Imam tried to negotiate a compromise.

After several days of fighting, the Imam and his family capitulated and were received well by Hülegü. Maymun-Diz was demolished, and the Imam ordered his subordinates to surrender and demolish their fortresses likewise. The subsequent capitulation of the symbolic stronghold of Alamut marked the end of the Nizari state in Persia.