In the past history of Iran, the province of South Khorassan has been a part of Ghahestan (meaning mountains in Arabic language) that was attributed to the southern part of Khorassan of which Qaen was the center.
In the legendary history of Iran, the foundation of Ghahestan has been attributed to Saam and Nariman in the time of Pishdadian Dynasty. There are some villages in the region called Salm-Abad, Fereydoon Geer and… indicative of this fact.
According to inscriptions belonging to Achaemenian time and some Greek historians, Ghahestan had been the habitat of Sagart and Aryan tribes. Herodotus, the Greek historian, describes that they have been oriental tribes and a part of 14th Satrap of Achaemenian Government.
In late 5th century AH, Ghahestan was an important center of Ismailieh sect under the leadership of Hassan Sabbah. Due to its natural conditions and backwardness, it was a suitable place for the activities of this movement after Alamoot in Ghazvin. In Safavid era, due to their policy for development of buisiness and commerce, this region developed considerably from economic ans security points of view. Then, Birjand, that was a village, was chosen as capital of Qaenat under the governorship of Alam Clan. This family played a very important role in all historical events of the region.
This province was a part of Khorassan Province till the year 2004 and it was separated as a new province.