Ubaydallah ibn Muhammad al-Mahdi () notes that he is sometimes mistakenly referred to as "Abdallah ibn al-Mahdi." (771–810/11) was an Abbasid prince. He was the son of al-Mahdi, the third caliph of the Abbasid Caliphate, and Raytah, daughter of the first Abbasid caliph Abu al-Abbas al-Saffah.; .
Life In 761, the future caliph al-Mahdi married Raytah as his first wife after his return from Khurasan. She was the daughter of al-Saffah and his wife Umm Salamah, a Makhzumite. Raytah gave birth to two sons, Ubaydallah and Ali.
During the reign of his half-brother Harun al-Rashid (), Ubaydallah was appointed as governor of Arminiyah and the northwestern provinces in 788/9, succeeding Yazid ibn Mazyad al-Shaybani.; . Numismatic evidence for this appointment is summarized by . Al-Ya'qubi () does not note Ubaydallah's governorship, saying instead that Ibn Mazyad was succeeded by Abd al-Kabir ibn Abd al-Hamid. Łewond (), claims that following a conflict between Harun and Ubaydallah the empire was split in two, with Ubaydallah receiving the northern provinces of Atropatene, Armenia, Iberia/Georgia, and Aghuania; this assertion is however disputed by , who notes that the Arabic sources from the period make no reference to any sort of conflict between the two brothers. , refers to Ubaydallah as a governor of the Jazira. He was later appointed to two brief stints as governor of Egypt, in 795 and 796.; ; (noting only one appointment to Egypt). During his first governorship he was placed in charge of both prayers/security (salah) and finances (kharaj); in his second administration he is mentioned as only being in charge of the salah.
In 810 or 811 Ubaydallah died in Baghdad. His nephew al-Amin led the prayers at his funeral.; .
Siblings Ubaydallah was contemporary and related to several Abbasid caliphs, princes and princesses. He had total ten half-siblings and he had one full brother named Ali ibn Muhammad al-Mahdi.
No.AbbasidsRelation1 Musa al-Hadi Half-brother2 Harun al-Rashid Half-brother3 Abbasa bint al-Mahdi Half-sister4 Ali ibn al-Mahdi Brother5 Mansur ibn al-Mahdi Half-brother6 Aliyah bint al-Mahdi Half-sister7 Ulayya bint al-Mahdi Half-sister8 Abdallah ibn al-Mahdi Half-brother9 Ibrahim ibn al-Mahdi Half-brother10 Banuqa bint al-Mahdi Half-sister11 Isa ibn al-Mahdi Half-brother
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