Abu l-Abbas al-Fadl ibn Sahl ibn Zadhanfarukh al-Sarakhsi (Arabic: أبو العباس الفضل بن سهل بن زادانفروخ السرخسي; Abu ’l-ʿAbbās al-Faḍl b. Sahl b. Zādānfarrūḫ as-Saraḫsī, died 818), titled Dhu 'l-Ri'āsatayn ("the man of the two commands"), was a famous Persian vizier of the Abbasid era in Khorasan, who served under Caliph al-Ma'mun (r. 813–832). He played a crucial role in the civil war between al-Ma'mun and his brother al-Amin (r. 809–813), and was the de facto ruler of the Caliphate until 817.
A Zoroastrian convert to Islam, he and his brother Hasan made many enemies while wielding incredible power in the courts of al-Mamun, particularly that he was suspected of being a Shia Alavid supporter. Ibn Athir reports that this fact (along with being a former al-majusi) led to considerable amounts of enmity from his Arab rivals, and eventually led to his murder by the vengeful Arabs in Sarakhs.
His brother was also eventually arrested, and al-Mamun moved his capital back to Baghdad from Merv.