This building is situated to the west of the Sanctuary of the Bona Dea (V,X,2). Along the west facade runs the Via di Felicissimo, that was blocked by the Baths of the Swimmer (V,X,3) to the south. The building has been dated to the Hadrianic period (opus mixtum). Pilasters against the outer east wall were added in the third century. The purpose of the building is uncertain. In the second room from the south the Mithraeum of Felicissimus (V,IX,1) was installed.