Al-Hasan ibn Zayd al-Da'i

Hasan ibn Zayd
Emir of Tabaristan
Silver dirham issued by Hasan
Reign	864–884
Successor	Muhammad ibn Zayd
Born	Medina
Died	6 January 884 Amol
Issue	One daughter (possibly)
House	Zaydid dynasty
Father	Zayd ibn Muhammad
Mother	Aminah bint Abdallah
Religion	Shi'a Islam

Al-Ḥasan ibn Zayd ibn Muḥammad ibn Ismā‘il ibn al-Ḥasan ibn Zayd (Arabic: الحسن بن زيد بن محمد‎‎; died 6 January 884), also known as al-Da‘ī al-kabīr (Arabic: داعي الكبير‎‎, "the Great/Elder Missionary"), was an Alid who became the founder of the Zaydid dynasty of Tabaristan.

Al-Ḥasan was a descendant of Hasan ibn Zayd ibn Hasan, a great-grandson of Ali, the son-in-law of Muhammad and fourth Caliph. In 864, he was living at Rayy in northern Iran, when he was invited by pro-Alid elements in the neighbouring province of Tabaristan to join them in an uprising against the Abbasid authorities.

Tabaristan, a mountainous region on the southern shore of the Caspian Sea, had remained largely untouched by the Muslim conquests of the 7th century. Until conquered by the Abbasid Caliphate in 759/60, it had been ruled by a native Persian dynasty, the Dabuyids, and even after the imposition of direct Muslim rule local dynasties retained a large measure of autonomy in the mountainous interior. It was only after 840, when Tabaristan came under Tahirid rule (the Abbasids' viceroys for the East), that the Islamization of the province began. It proceeded rapidly, and although the majority of the people adopted Sunni Islam, the province offered opportunities for the activities of pro-Alid Shi'ite missionaries as well.