Ali ibn Musa al-Rida (Arabic: عَلِيّ ٱبْن مُوسَىٰ ٱلرِّضَا, romanized: ʿAlī ibn Mūsā al-Riḍā, c. 1 January 766 – 6 June 818), also known as Abū al-Ḥasan al-Thānī, was a descendant of the Islamic prophet Muhammad, and the eighth imam in Twelver Shia Islam, succeeding his father, Musa al-Kazim. He is also part of the chain of mystical authority in Sunni Sufi orders. He was known for his piety and learning, and a number of works are attributed to him, including Al-Risala al-Dhahabia, Sahifa al-Rida, and Fiqh al-Rida. Uyun al-Akhbar al-Rida by Ibn Babawayh is a comprehensive collection that includes his religious debates and sayings, biographical details, and even the miracles which have occurred at his tomb. He is buried in Mashad, Iran.

Al-Rida was contemporary with the Abbasid caliphs Harun al-Rashid and his sons, al-Amin and al-Ma'mun. In a sudden departure from the established anti-Shia policy of the Abbasids, possibly to mitigate the frequent Shia revolts, al-Mamun invited al-Rida to Marv in Khorasan, his de facto capital, and designated him as heir apparent, despite the reluctance of the al-Rida who accepted the offer on the condition that he would not interfere in governmental affairs. The appointment of the Ali al-Rida by the Abbasid al-Mamun immediately invoked strong opposition, particularly among the Abbasids and Arab Sunni nationalists, who revolted and installed Ibrahim al-Mubarak, a half-brother of Harun al-Rashid, as the anti-caliph in Baghdad. Realizing the severity of the Iraqi opposition, al-Mamun and his entourage left Khorasan for Baghdad, accompanied by al-Rida. The Imam, however, died mysteriously when the party reached Tus in September 818. His death followed shortly after the assassination of al-Fadl ibn Sahl, the Persian vizier of al-Mamun, who was publicly seen as responsible for his pro-Shia policies. The caliph is often seen as responsible for both deaths, as he made concessions to the Arab party to smooth his return to Baghdad. Tus was later replaced with a new city, called Mashhad, which developed around the grave of al-Rida as the holiest site in Iran, to which millions of Shia Muslims flock annually for pilgrimage.