The Masjid-i Jami of Fahraj is the congregational mosque of the town of Fahraj, thirty kilometers east of the city of Yazd. The mosque is significant as one of the oldest extent mosques in Iran, representing an important evolutionary stage in mosque construction from post and lintel systems, to wooden roofs on arcades, finally to full scale vaulting with permanent materials. The Masjid-i Jami of Fahraj along with the Tarik Khana mosque at Damghan marks the change of the mosque construction's primary structural member from columns to piers, in the ninth century. The original patron and date of construction cannot be verified, but a date in the Sassanid period cannot be ruled out considering the excavation of ancient Zoroastrian references and the prevalent tradition of appropriating sacred sites. Fahraj lies on the old route between Yazd and Bafq and was an important urban settlement in Sassanid times, known by various names such as Mihrpadeen, Mihreez, Fahrashan or Pahreh. Iranshahr, today the largest city in the state of Baluchistan in Iran was also referred to as Fahraj in the ancient Greek records of Alexander's campaigns.
The 
mosque primarily consists of an internal courtyard, vaulted sanctuary and arcades, and a clay minaret. It is largely built of sun-dried, unfired clay tiles and mud bricks. Its modest internal courtyard is lined with clay tiles and contains a now dry central ablution tank. The internal facades are near symmetrical and consist of three arched bays defined by thick piers, partly relieved by vertical niches. The court's roofline is uninterrupted and a decorative cornice emphasizes horizontality that is challenged by the mosque's minaret and arched silhouette of barrel vaults over the jamaat khana (main prayer hall). The mosque, like its contemporaries at Damghan and Nayin, features the traditional Iranian barrel vault (or beez), supported on squat rectangular brick piers. The roofing system is made up of five linear vaults oriented east-west, three of which are interrupted by the central courtyard. Two additional vaults orient north-south, and the roof extends to the mosque's northwest corner. The minaret added to the mosque later in the tenth century is also one of the earliest extant examples of its type, with the minaret at Nayin. The minaret is built as a tapering cylinder with an internal spiral staircase lit by slits that form a regular pattern on the minaret's external elevation. The projecting balcony is crowned by triangular crenellations with a pronounced batter (a slope, as of the outer face of a wall, that recedes from bottom to top), accentuating the minaret's verticality. The Fahraj minaret is one of the first known Iranian minarets after the minaret at Siraf, and has played a pivotal role in developing the cylindrical minaret form, that was later carried far with Seljuk conquests to Syria, Anatolia, Iraq, Afghanistan and India.
The mosque design's simplicity and lack of ornamentation have denied it the public attention and preservation efforts deserved by a building emblematic of so pivotal a stage in Iranian Islamic architecture and heritage. The mosque represents a little documented innovation in Islamic architecture that was soon lost to the dominant four-iwan prototype patronized by the Seljuks in the eleventh century.