Masjid-i Imam (formerly Masjid-i Shah) is a congregational mosque located in the historic bazaar of Tehran. Fath 'Ali Shah Qajar commissioned the building, which is dated to 1824/1240 AH in an inscription.

The mosque is a rectangular mass with a large courtyard at its center oriented northeast-southwest toward the qibla. A large rectangular plaza planted with two strips of trees leads visitors to a grand iwan portal vaulted with muqarnas. This portal gives onto a domed vestibule, which in turn gives onto the angled back wall of an iwan. This iwan opens onto the northeastern side of the mosque's large central courtyard. The back wall is angled to conform to the north-south axis of the entrance portal and plaza, while the front of the iwan conforms to the northeast-southwest orientation of the courtyard and prayer halls.

An iwan opens at the center of each of the courtyard's four walls. The aforementioned northeastern iwan, which connects the entrance to the courtyard, is ornately decorated, featuring two minarets and a clock tower. The iwan opposite it on the southwestern side of the courtyard is the qibla iwan, which also features an ornate facade that rises above the height of the surrounding walls. The iwans on the lateral (southeast and northwest) sides of the court are shorter than the qibla and entrance iwans, and are both flanked by smaller archways surmounted by second story balconies. The spaces between the four iwans are bridged by arcades of broad pointed arches with glass windows and doors that open onto interior vaulted spaces around the courtyard. On the courtyard's two lateral sides, these spaces are only one domed bay deep.

The qibla iwan opens onto a large domed chamber with a mihrab in its southwestern wall. On either side of this iwan-dome structure are hypostyle prayer halls of five aisles parallel to the qiblal, each four bays long. Each bay is covered by a small dome. A third prayer hall is located at the northeastern corner of the mosque behind the row of vaulted spaces surrounding the courtyard. It is four aisles deep and four bays wide.

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