This study is an attempt to re-construct the history of the town of Hadjar, probably identical with antique Gerra / Gerrha, in Sasanid times. The history of ancient Gerra / Gerrha ended in the second half of the 2<sup>nd</sup> century, when its inhabitants were driven away by the Arabic tribes of Tanūkh. Sasanids established their rule over Hadjar in the 1<sup>st</sup> half of the 3<sup>rd</sup> century and kept it till the late 5<sup>th</sup> century. The advance of King Abkarib As‘ad of Himyar and Saba to the north in the mid-5<sup>th</sup> century affected Hadjar and probably resulted in a clash with Sasanid forces, which was reflected in Arabic legends as his victory over Iran. However, Hadjar seems to have remained under the Sasanid power. Then, from the late 5<sup>th</sup> through the end of the 6<sup>th</sup> century Hadjar passed from Sasanids and their vassals, the Lakhmid kings of al-Ḥīra, to Banū Kinda and back. It was finally recovered by the Sasanids at the end of the 6<sup>th</sup> century, following the migration of Banū Kinda from the Baḥrayn region. The Sasanids re-constructed and fortified Hadjar. By the beginning of the 7<sup>th</sup> century Hadjar was an enough strong fortress to be selected as the place of the attack on Banū Tamīm. Like al-Ḥīra, Hadjar hosted then two governors, one Persian and one Arab. Economically, in the late 6<sup>th</sup> through the early 7<sup>th</sup> century Hadjar was an important centre of trade. In the 6<sup>th</sup> century and later on, a Christian community headed by a bishop was present in Hadjar.