Ali al-Rida (Arabic: عَلِيّ ٱبْن مُوسَىٰ ٱلرِّضَا, romanized: ʿAlī ibn Mūsā al-Riḍā, c. 1 January 766 – c. 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 in 799 CE. 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-Risalah al-Dhahabiah, Sahifah of al-Ridha, and Fiqh al-Rida. Uyoun Akhbar Al-Ridha 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, site of a large shrine.

Al-Rida was contemporary with the Abbasid caliphs Harun al-Rashid (r. 786–809) and his sons, al-Amin (r. 809–813) and al-Ma'mun (r. 813–833). 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 Merv 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 Ali al-Rida by the Abbasid al-Mamun immediately invoked strong opposition, particularly among the Abbasids, who revolted and installed Ibrahim ibn al-Mahdi, a half-brother of Harun al-Rashid, as the anti-caliph in Baghdad. Realising 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.

Ali was born in Medina in 148 AH (765/766 CE), 151 AH (768/769CE), or 153 AH (769/771CE). The first date is said to be based on a prediction ascribed to his grandfather, al-Sadiq, who died in that year, that the successor to his son al-Kazim would be born soon. There are some indications that Ali might have been born as late 159 AH (775/776CE). In any case, the date often given by Shia authorities is 11 Du al-Qa'da 148 AH (765/766CE). His father was al-Kazim, the seventh Twelver Shia Imam, who was a descendant of Ali and Fatima, cousin and daughter of the Islamic prophet, respectively. His mother was a freed slave, probably of Berber origin, whose name is recorded differently in various sources, perhaps Najma or Tuktam. It was reputedly Ja'far al-Sadiq, the father of al-Kazim, who chose Najma for him. Momen writes that Ali was thirty-five years old when his father died, whereas Donaldson holds that he was twenty or twenty-five at the time.

Al-Rida lived with his father Musa al-Kazim in Medina until 179 AH (795/796CE). When Harun arrested Musa and transferred him to Iraq, he took care of Musa's property and the affairs of the Shias as the attorney of his father. With the death of Musa al-Kazim in Baghdad prison in Rajab 183 AH (799/800CE), his son Ali al-Rida became his heir and successor, according to his father's will.

According to Madelung, al-Kazim had appointed al-Rida as his executor and al-Rida also inherited his father's property near Medina, excluding his brothers. According to Musa al-Kazim will, the custody of his children, wives and property was also entrusted to Ali al-Rida. Ali spent the next ten years of his life - from 183 AH (799/800CE) to 193 AH (808/809CE) - in the reign of Harun.

The Abbasid caliph Harun died during the imamate of al-Rida and the empire was split between his two sons: the reigning caliph, al-Amin, who was born to an Arab mother, and al-Mamun, who was born of a Persian mother and was designated as the successor and the governor of the province of Khorasan in present-day Iran. In effect, according to Momen, al-Amin controlled Iraq and the west with his Arab vizier, al-Fadl ibn Rabi, while al-Mamun controlled Iran and the east with his Persian vizier, al-Fadl ibn Sahl. Al-Amin reportedly violated these arrangements by appointing his son as successor in place of Mamun, and soon a civil war ensued in which al-Amin was killed and Baghdad was occupied by al-Mamun's general, who nevertheless remained in Merv in Khorasan, apparently determined to make there his new capital. Al-Rida, who, according to a tradition narrated by Ibn Babawayh and al-Tabarsi, had informed one of his companions that Ma'mun would kill Amin, kept himself away from the conflicts between the two. The period of Amin's caliphate is said to be the time of peace for al-Rida, during which he found the opportunity to fulfill his mission of spreading the Islamic teachings.

Al-Mamun claimed for himself the title of Imam al-Huda (lit. 'rightly-guided leader'), possibly to imply that he was best qualified for the caliphate. Notably, he faced costly revolts in Kufa and Arabia by Alids and Zaydis, who intensified their campaign against the Abbasids around 815 CE, seizing the cities of Mecca, Medina, Wasit, and Basra. In particular, the Shia revolt by Abu'l-Saraya in 815 was difficult to suppress in Iraq, and compelled al-Hasan ibn Sahl, al-Mamun's governor of Iraq, to deploy the troops of the Khorasani general Harthama.

Throughout the years, several of al-Rida's brothers and his uncle Moḥammad ibn Ja'far participated in the Alid revolts in Iraq and Arabia, but al-Rida refused any involvement. In this period, al-Rida's only involvement in politics might have been to mediate between the Abbasid government and his uncle Muḥammad ibn Ja'far, who had revolted in Mecca.