Abu'l-Husayn al-Qasim ibn Ubayd Allah (Arabic: أبو الحسين القاسم بن عبيد الله‎) was a senior official of the Abbasid Caliphate who served as vizier from April 901 until his own death in October 904. Hailing from the Banu Wahb, a family of Nestorian Christian origin that had served in the caliphal bureaucracy since late Umayyad times, Ubayd Allah was the son and grandson of viziers. He had served as aide to his father, Ubayd Allah ibn Sulayman, during the latter's decade-long vizierate, and then succeeded him upon his death, heading the government during the last months of the reign of al-Mu'tadid (r. 892–902) and the early years of al-Muktafi (r. 902–908). Al-Qasim largely dominated the young al-Muktafi, who awarded him with the title of Wali al-Dawla ("Protector of the Dynasty") and gave