Isma'il ibn Ja'far (Arabic: إسْماعِيل ٱبْن جَعْفَر ٱلْمُبَارَك, romanized: Ismāʿīl ibn Jaʿfar al-Mubārak) was the eldest son of Ja'far al-Sadiq and the sixth Imam in Isma'ilism. He carried the epithet of al-Mubarak, on the basis of which one of the earliest Isma'ili groups became designated as the Mubarakiyya.

It seems likely that the Mubarakiyya were originally supporters of Isma'il before acknowledging Muhammad ibn Isma'il as their Imam. At any rate, Mubarakiyya was thus one of the original names of the nascent Isma'iliyya, a term coined by later heresiographers. A faction of the Mubarakiyya later developed into the Fatimid Isma'ilis, upholding the continuity of the Imamate in the progeny of al-Mubarak, acknowledging al-Mubarak himself as their sixth Imam. This enumeration was subsequently retained by the various branches of the Isma'ili.

A major crisis arose among the Shia after the death of Ja’far al-Sadiq, who had five sons. Abd Allah al-Aftah and Isma'il al-Mubarak were the eldest sons by his first wife Fatima, a granddaughter of Hasan Ibn Ali. Al-Mubarak was probably the second son of al-Sadiq. The exact date and circumstances of al-Mubarak's death also remain obscure. According to some Isma'ili authors, al-Mubarak survived al-Sadiq. Some sources, mainly Twelver texts, report that al-Mubarak died during the lifetime of al-Sadiq—but those same sources also report that al-Mubarak was seen several days later in Basra, suggesting that he did not really die but was sent away out of Medina.

Information about the life of Isma'il al-Mubarak, his connections and the movement carried out by his followers is extremely rare. Among the early works, only a few Twelver sources give some information about the subject. However, due to open rivalry between the Isma'ilis and the Twelvers, the information derived from the Twelver books always carries the risk of being one-sided and thus its authenticity is open to doubt. On the other hand, early Isma'ili sources give almost nothing for this period. As the Islamicist Farhad Daftary observes, because of being religious and philosophical in their character contain little historical information on the initial period of the sect, and those which have information, because of the views contained in them were in conflict with the official Fatimid doctrines, probably were subjected to later censorship by the Fatimid Isma'ilis.

An important Isma'ili work is Uyūn al-Akhbār written by Idris Imad al-Din (d. 1468), an Isma'ili da'i in Yemen. One volume of this work contains some important historical material relating to al-Mubarak and his son Muhammad. According to the orientalist Vladimir Ivanov (d. 1970), Idris quoted this information literally from an early work, Sharḥ al-Akhbār fī Faḍā'il al-A'imma al-Aṭhār, written by the Isma'ili jurist and theologian Abu Hanifa Nu'man ibn Muhammad al-Tamimi, who is well-known as al-Qadi al-Nu'man (d. 974). Another work of Idris is Zahr al-Ma'ānī; it gives a review of the Isma'ili Imams. The third Isma'ili work is Asrār al-Nuṭaqā' written by Ja'far ibn Mansur al-Yaman, a contemporary of al-Qadi al-Nu'man. The main subject of the book is the story of Abraham, and the proofs of the right of al-Mubarak to the Imamate. As Ivanov says, it contains a strong controversial element directed against the Twelvers.

Isma'il ibn Ja'far ibn Muhammad al-Mubarak's birth date is unknown, but apparently he was the second son of Ja'far al-Sadiq, born between 80 and 83/699–702. His mother, Fatima bint al-Husayn al-Athram ibn al-Hasan ibn Ali, was the first wife of al-Sadiq. Her mother was Asma, daughter of Aqil ibn Abi Talib. The Sunni theologian Muhammad al-Shahrastani (d. 1153) writes in his Kitāb al-Milal wa al-Niḥal that during the lifetime of Fatima, al-Sadiq never got another marriage like Muhammad with Khadija and Ali with Fatima. Al-Mubarak was the full-brother of Abd Allah al-Aftah, the Imam of the Fathiyya (or Aftahiyya).

The early life of al-Mubarak is obscure except few fragmented records. He was alive in 133/751, when Dawud ibn Ali, the governor of Medina, executed Mu'alla ibn Khunays probably because of his agency on behalf of al-Sadiq and his involvement in some revolutionary activities of Muhammad al-Nafs al-Zakiyya, al-Sadiq, as soon as he heard the news of the execution of Mu'alla, went to the governor with his son al-Mubarak and called him to account for this murder. The governor Dawud put the blame on his chief of shurta (the chief of his police force), al-Sayrafi. According to the Twelver al-Kashshi (d. 978), al-Sadiq directed al-Mubarak to kill al-Sayrafi, which he did. In Idris' report, al-Sadiq then handed him over to al-Mu'alla's associates and they killed him.

Riyah ibn Uthman al-Murri, the Abbasid governor in Medina burnt the house of Ahl al-Bayt, and al-Mubarak was decided to be killed. Ahmad ibn Ali al-Najashi (d. 1058) writes in his Kitāb al-Rijāl, that al-Mubarak was involved in a militant anti-Abbasid plot in collaboration with several others, which included Bassam ibn Abdullah Sayrafi, a radical Shia engaged in money lending in Kufa. Bassam was brought out dead; Isma'il was spared probably due to his father's status. Muhammad Hasan al-Muzaffar (d. 1955) quotes al-Sadiq as saying that, "Isma'il was planned two times for killing, but I prayed for his life, and God protected him." This association is one of several reported by the Twelver sources which caused al-Sadiq to express his dissatisfaction with those radical Shias who were leading his son astray.