Ja'far ibn Abdallah al-Mansur (Arabic: جعفر بن عبد الله المنصور) was the elder son of second Abbasid caliph al-Mansur and elder brother of third Abbasid caliph al-Mahdi.
As the elder son of al-Mansur, he was the candidate to the caliphal throne until his premature death.
Ja'far was the son of Abdallah ibn Muhammad al-Abbasi al-Hashimi (the future caliph al-Mansur) in 742/743.
[3] Arwa had two sons, Muhammad (future Caliph al-Mahdi) and Ja'far.
Jafar also had several half-siblings, After his mother's death, his father remarried and had children from his wives.