Sanjan was an ancient city in Greater Khorasan, a realm in northeastern Iran in the vicinity of the historically eminent city of Merv. Sanjan's location in historical Greater Khorasan is at present a geographic part of Turkmenistan. Notwithstanding its present ethnic population, Sanjan and in particular Merv have for centuries been regarded as the cradle of the Aryans, in the East, and the epitome of Persian culture. A great lot of Indian Parsees, descendants of Iranian Zoroastrians, who fled Islamic Arab conquerors in the seventh and eighth centuries, trace their origin to the city of Sanjan.

Sanjan must not be confused with the phonetically similar Iranian province Zanjan, or its capital, the city of Zanjan.