Tayy (/ ALA-LC: Ṭayy), also known as Ṭayyiʾ, is a large and ancient Arab tribe, whose descendants continue to live throughout the Middle Eastern states of the Arab world. The nisba ( patronymic) of Tayy is at-Ṭaʾī. The Tayy's origins trace back to the Qahtanites and their original homeland was Yemen. In the 2nd century CE, they migrated to the northern Arabian mountain ranges of Jabal Aja and Jabal Salma where they became close allies of the Banu Assad tribe. The mountains, which became known as "Jabal Tayy", thereafter served as the traditional homeland of the Tayy and its later subbranches until the present day. They later established relations with the Sassanid Persian and Byzantine empires, which ruled Mesopotamia and Syria, respectively. While traditionally allied with the Sassanids' Lakhmid clients, in the first decade of the 7th century, the Tayy supplanted the Lakhmids as the provincial rulers of al-Hirah (near Kufa) on behalf of the Persians. In the late 6th century, the intra-tribal Fasad War split the Tayy, with members of its Al Jadila branch converting to Christianity and migrating to Syria where they became allied with the Ghassanids (Arab clients of the Byzantines), and the Al Ghawth branch remaining in Jabal Tayy. A chieftain of the Al Ghawth, Hatem at-Ta'i, emerged during this time as a poet widely known among Arabs until today for his generosity.
Hatem's son Adi, and another leading Tayy tribesman, Zayd al-Khayr, converted to Islam together with much of their tribes in 629–630, and became companions of the Islamic prophet Muhammad. The Tayy participated in numerous Muslim military campaigns after Muhammad's death, particularly during the Ridda Wars against pagan Arab tribes and the Muslim conquest of Persia. The Al Jadila in northern Syria remained Christians until shortly after the Muslim conquest of their region in 638. The Tayy was split during the First Fitna, with those based in Arabia and Iraq supporting Ali as caliph and those in Syria supporting Mu'awiyah. The latter and his Umayyad kinsmen ultimately triumphed and members of the Tayy participated in the Umayyad conquest of Sindh in the early 8th century. Nonetheless, a branch of the Tayy under Qahtaba ibn Shabib were among the leaders of the Abbasid Revolution which toppled the Umayyads in the mid-8th century. The Tayy fared well under the Abbasid Caliphate, producing military officials and renowned poets, such as Buhturi and Abu Tammam.
By the mid-9th century, Abbasid authority had eroded and the Tayy were left dominant in the southern Syrian Desert and Jabal Tayy. Under their Jarrahid chieftains, they established themselves in Palestine under Fatimid rule. As the virtually independent rulers of the area between al-Ramla and Jabal Tayy, they controlled the key routes between Egypt, Syria, Arabia and Iraq. They vacillated between the Fatimids and the Byzantines and then between the Seljuks and Crusaders until the late 12th and early 13th centuries, when the Tayy's various subbranches, chief among them the Al Fadl, were left as the last politically influential Arab tribe in the region extending from Najd northward to Upper Mesopotamia.