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.^[1] 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 (Arabic: ولي الدولة, lit. 'Protector of the Dynasty') and gave one of his daughters to one of al-Qasim's sons.^[2]

Unlike his father, who was widely esteemed for his honesty and justice, al-Qasim was corrupt and cruel, ordering the executions of anyone that displeased him or presented a potential challenge, such as the Saffarid emir Amr ibn al-Layth, the general Badr al-Mu'tadidi, or the poet Ibn al-Rumi. The powerful finance secretary Ali ibn al-Furat was saved from a similar fate only by al-Qasim's illness and death. This death meant the end of the Banu Wahb's hold on power, which now passed to the Banu'l-Furat. Only a generation later would al-Qasim's sons al-Husayn and Muhammad also rise to become viziers.^[2]