The Bab al-Masjid Husayniyah is located in the old city of Na’in, Iran, adjacent to the city’s Friday Mosque, from which it and the surrounding area derive their name. The Husayniyah dates to the Qajar period. Its core comprises an open square (maydan) and roofed hall (husayniyah sarpushidah). Adjoining these and associated with them are a domed tomb with the graves of several mystics, including Mullah Ni’mat-Allah Nistanki; a second tomb known as Maqbarah-i Pir; a cistern (ab anbar); and a water dispenser (saqqa-khanah). 

The husayniyah's open space is a rectangular public square oriented roughly north-south. The square is accessible through three passageways leading from neighborhood streets: one in the northwestern corner, one in the southeastern corner, and one in the southwestern corner. The shorter north and south facades of the square have nearly identical organizations: at the center of each, a covered seating area (shah-nishin) opens onto the square. These take the form of an arched iwan elevated several feet from the level of the square and rising two stories in height. Flanking the seating areas on each side are a narrow archway that houses a staircase leading to the seating area, and a broader archway. These rise one story and are surmounted by balconies of similar form. On the longer west and east sides of the square, a two-story iwan shallower than those on the north and south sides opens in the center and is flanked by two sets of broader archways surmounted by balconies.

Directly behind the seating area on the northern side is a domed tomb chamber. The covered husayniyah hall adjoins these to the east, and is accessible through the outer archway on the northern facade of the square closest to its northeastern corner.

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