South Khorasan is a province located in eastern Iran. Birjand is the centre of the province. The other major cities are Ferdows and Qaen. Greater Khorasan has witnessed the rise and fall of many dynasties and governments in its territory throughout history. Various tribes of the Arabs, Turks, Mongols, Turkemen and Afghans brought changes to the region time and time again. Ancient geographers of Iran divided Iran ("Iran-Shahr") into eight segments of which the most flourishing and largest was the territory of Greater Khorasan.

South Khorasan, province of Iran consists of 7 counties: Birjand, Ferdows, Qaen, Sarayan, Nehbandan, Darmian and Sarbisheh. The famous Parthian empire was based near Merv in Khorasan for many years. However they did have many fortifications and strongholds in Khorasan, such as the Nehbandan castle in South Khorasan province. During the Sassanid dynasty the province was governed by an Espahbod (Lieutenant General) called "Padgoosban" and four margraves, each commander of one of the four parts of the province.