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. When Ja'far was eleven or twelve year old, his father became caliph.

His mother was Arwa. She was al-Mansur's first wife. Arwa also known as Umm Musa, whose lineage went back to the kings of Himyar.^[2] Her father was Mansur al-Himyari. She had a brother named Yazid.^[3]

Arwa had two sons, Muhammad (future Caliph al-Mahdi) and Ja'far.^[2] She died in 764.^[2] The two sons of Arwa, Ja'far and Muhammad were regarded as his heirs after the removal of Isa ibn Musa as heir. However Ja'far died during his father al-Mansur's reign. His brother, Muhammad became sole candidate to the throne.

One of his al-Mansur's concubines was a Kurdish woman. She was the mother of al-Mansur's son Ja'far.^[3] Al-Mansur named this son after his elder son.

After Ja'far's death, his father too care of his young children. When his father, al-Mansur died. His brother caliph al-Mahdi took the responsibility of his children. Al-Mahdi married his son and heir to Ja'far's daughter Zubaidah.

Ja'far was related to several powerful figures of Abbasid House. He was the older brother of al-Mahdi and he was also maternal grandfather of al-Amin.

Jafar also had several half-siblings, After his mother's death, his father remarried and had children from his wives.