Tahir ibn Husayn (Arabic, Persian: طاهر بن حسين) (died 822) was a general and governor during the Abbasid caliphate. Specifically, he served under al-Ma'mun during the Fourth Fitna and led the armies that would defeat al-Amin, making al-Ma'mun the caliph. He was born in Phoshang which is a village in ancient city of Herat (then Khorasan present day Afghanistan).
Afterwards, Tahir was made governor of the eastern Abbasid lands, effectively making him governor of Persia. Tahir later declared independence from the Abbasid empire in 822 by omitting any mention of al-Ma'mun during a Friday sermon. However, he died the same night and al-Ma'mun appointed Tahir's son to continue at his father's post. This established the Tahirid dynasty, which ruled a semi-autonomous state in eastern Persia.
Tahir commissioned the Christian theologian, Theodore Abu-Qurrah (died c. 830) to translate the pseudo-Aristotelian De virtutibus animae into Arabic.