This mithraeum was found in the west part of the Imperial Palace. It was excavated in 1860-1861 by C.L. Visconti. It measures c. 16.75 x 5.30. The west wall is of opus mixtum, the south wall of opus latericium with a door (w. 1.15), the north wall of small tufa blocks. The east wall consists of five brick piers, between which are walls of latericium and small tufa blocks. In the south part of the east wall is a window (h. 0.90, at a height of 2.20), below which is a square wall-niche (0.40 x 0.40, d. 0.50). All walls were decorated with red plaster without any motifs.

In the south-east corner is a square niche on a base. In the base is a hollow space in which lamps were found. A tufa base is standing against the centre of the north wall. It is an altar, or carried a niche. The front part of the base has five marble treads, on top of which is a masonry base carrying a marble altar with the inscription:

Near the base a few small marble columns were found, presumably lamp-supports, and some tufa fragments that imitate the rock from which Mithras was born (petra genetrix).

The podia were reached along a few treads at the south end. In the vertical side, halfway down the shrine, are niches for small statues of Cautes and Cautopates (now in the Vatican Museums). On the bases of the statues are reliefs of Cautes and Cautopates, with the inscription:

and on one of the bases, on the side:

On the floor is a white mosaic with an identical text running along the podia:

Other finds are: several lamps, one with holes for twelve wicks; a statuette of Cautopates; a portrait head of Mithras with remains of paint, and his right hand with a knife; a Phrygian cap with holes for rays; the head of a lion, a reference to the grade Leo.

In a nearby room a mosaic of Silvanus, dated to the period of Commodus, was found in a niche. It may well have been related to the shrine.