| Ubaydallah ibn al-Mahdi عبيد الله بن محمد المهدي | |
| Succession: | Governor of Arminiya |
| Reign: | c. 788 – 791 |
| Reign-Type: | In office |
| Predecessor: | Yazid ibn Mazyad al-Shaybani |
| Successor: | Abd al-Qadir (791) then Al-Fadl ibn Yahya |
| Succession1: | Governor of Egypt |
| Reign1: | 795 – 795 (first term) |
| Reign-Type1: | In office |
| Predecessor1: | Abdallah ibn al-Musayyab |
| Successor1: | Musa ibn Isa |
| Reign2: | 796 – 797 (second term) |
| Reign-Type2: | In office |
| Predecessor2: | Musa ibn Isa |
| Successor2: | Isma'il ibn Salih |
| Dynasty: | Abbasid |
| Mother: | Raytah |
| Birth Date: | 771 |
| Birth Place: | Baghdad, Abbasid Caliphate |
| Death Date: | 810/11 |
| Death Place: | Baghdad, Abbasid Caliphate, now Iraq |
| Full Name: | Ubaydallah ibn Muhammad al-Mahdi ibn Abdallah al-Mansur |
| Burial Place: | Baghdad |
| Religion: | Islam |
Ubaydallah ibn Muhammad al-Mahdi (Arabic: عبيد الله بن محمد المهدي|ʿUbayd Allāh ibn Muḥammad al-Mahdī)[1] (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.[2]
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.[3] He was later appointed to two brief stints as governor of Egypt, in 795 and 796.[4]
In 810 or 811 Ubaydallah died in Baghdad. His nephew al-Amin led the prayers at his funeral.[5]
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. | Abbasids | Relation | |
|---|---|---|---|
| 1 | Musa al-Hadi | Half-brother | |
| 2 | Harun al-Rashid | Half-brother | |
| 3 | Abbasa bint al-Mahdi | Half-sister | |
| 4 | Ali ibn al-Mahdi | Brother | |
| 5 | Mansur ibn al-Mahdi | Half-brother | |
| 6 | Aliyah bint al-Mahdi | Half-sister | |
| 7 | Ulayya bint al-Mahdi | Half-sister | |
| 8 | Abdallah ibn al-Mahdi | Half-brother | |
| 9 | Ibrahim ibn al-Mahdi | Half-brother | |
| 10 | Banuqa bint al-Mahdi | Half-sister | |
| 11 | Isa ibn al-Mahdi | Half-brother |