Imam Reza (A.S.) left behind only one son, Imam Muhammad al-Taqi (A.S.), who was born on 10th Rajab in 195 AH (811 AD) . His mother's name was Abu Jafar, and two of his most famous epithets were Taqi and Jawad. At the time of his birth in Medina Muhammad Amin was the ruler in Baghdad.

Following Imam Reza's (A.S.) nomination as heir, Mamun had engaged his young daughter Umm al-Fazl to Imam Taqi (A.S.). The 9th Imam was in his 9th year at the time of his father martyrdom, and with his God-gifted spiritual powers came from Medina to Khorasan for the last rites of Imam Reza (A.S.). The period of his Imamate was 18 years.

Imam Taqi's (A.S.) wife, Umm al-Fazl was ultimately responsible for her husband's death she instigated her uncle Mu'tasem (the new caliph) to poison Imam Taqi (A.S.). Thus was martyred the 9th Imam in Baghdad in the year 220 AH (835 AD), and was laid to rest beside his grandfather Imam Musa al-Kazem in the holy shrine in Baghdad which is known today as Kazemain.