ʿAbdallāh al-Afṭaḥ ibn Jaʿfar al-Ṣādiq (Arabic: عبدالله الافطح بن جعفر الصادق, d. 766 CE / 149 A.H.)^[1] was the eldest son of Ja'far al-Sadiq (after al-Sadiq's death) and the full-brother of Isma'il ibn Jafar.^[2] Abdallah's title "al-Aftah" derives from the Arabic words "aftah al-ra’s" (broad-headed) or "aftah al-rijlayn" (broad-footed) used to describe his appearance.^[3]

During the lifetime of his father, Abdallah al-Aftah had supported the revolt of his relative Muhammad al-Nafs al-Zakiyya.^[4]

Following Ja'far al-Sadiq's death, the majority of Ja'far's followers accepted Abdallah al-Aftah as their new Imam. These followers were known as the Fathites and, according to the Mu'tazili heresiographer Abul-Qasim al-Balkhi al-Ka‘bi (d.319 A.H. / 931 CE), they were the biggest and most important section of the followers of Ja'far al-Sadiq.^[5] To support his claims, Abdallah al-Aftah seems to have claimed a 2nd Nass from his father (following Ismā'īl's demise) and his adherents cited a supposed Hadith from Ja'far al-Sadiq to the effect that the Imamate must be transmitted through the eldest son of the Imam. However, when Abdallah al-Aftah died about in 766 after the death of his father, the bulk of his supporters went over to his brother Musa al-Kazim.^[6] Other Fathites considered Abdallah al-Aftah the 7th Imam and Musa al-Kazim the 8th Imam,^[7] while others believed the Imamate came to an end when Abdallah al-Aftah died.^[5] Another group invented a son for Abdallah al-Aftah, called Muhammad ibn Abdallah al-Aftah, because they unconditionally believed the Imamate could only be inherited from father to son, rather than from brother to brother. This group also claimed that Muhammad ibn Abdallah al-Aftah was the promised Mahdi.

In a letter sent to the Isma'ili community in Yemen by the first Fatimid caliph, Abdallah al-Mahdi Billah, which was reproduced by Ja'far ibn Mansur al-Yaman, Abdallah al-Aftah was referred as Sāhib al-Haqq or the legitimate successor of Ja'far al-Sadiq, in an attempt to explain the genealogy of his ancestors. Instead of tracing his descent to Isma'il ibn Jafar and his son Muhammad ibn Ismail, al-Mahdi Billah designated al-Aftah as his forefather. According to al-Mahdi Billah, al-Aftah had called himself 'Isma'il ibn Ja'far' for the sake of taqiyya, and each of his successors had assumed the name Muhammad. Al-Mahdi Billah explained the genealogy of the Fatimid Caliphs and claimed Fatimid ancestry by declaring himself to be ʿAli ibn al-Ḥusayn ibn Aḥmad ibn ʿAbadullāh ibn ʿAbd Allāh ibn Jaʿfar al-Sadiq. But the Imamah (Ismaili doctrine) was later formulated in a different manner since ʿAbdallah al-Mahdi Billah's explanation of his ancestry was not accepted by his successors.^[8]

• Descendants of Ali ibn Abi Talib