In 1959, during dredging in the part of the Fossa Traiana near the ancient beach, a marble architrave was found belonging to a temple of Isis (AE 1968, 86; now in the Museo Nazionale in Rome). An inscription on the architrave informs us that the temple was restored as late as the years 375-378 AD by the Emperors Valens, Gratianus and Valentinianus:
DDD(omini) NNN(ostri) VALENS GRATIANVS ET VALENTINIA[nus Auggg(usti)]
AEDEM AC PORTICV[m] DEAE ISIDI RESTITVI PRAECEPERUNT
CVRANTE SEMPRONIO FAVSTO V(iro) C(larissimo) PRAEFECTO ANNONAE
"Our three lords Valens, Gratianus and Valentinianus, Augusti, decided to restore the temple and the porticus for the goddess Isis, with Sempronius Faustus, the very famous man, prefect of the grain-supply, as supervisor of the work".
It might be this temple that is mentioned in two other inscriptions from Portus, from the late second or third century (CIL XIV, 18 and 19). Both were found in 1868, but the exact place of discovery is not known. One records a restoration by a priest of Isis and the Isiaci, the other mentions an enlargement by two women. Both inscriptions mention some sort of (underground?) hall, a megaron.
[Pr]O SALVTE IMP(eratoris) CAES(aris)
///// P(ii) F(elicis) A
VG(usti) CAMVRENIVS VERV(s) SAC(erdos)
DEAE ISIDIS CAP(itolinae)
CED ET CETERI
[Isi]ACI MAGAR(um) DE SVO RESTITV(erunt)
ET AVRELIA SEVERA
On what was in antiquity the north-west corner of the Isola Sacra, the remains were found of what has been called the Temple of Isis Pelagia, protective deity of shipping. Excavations were carried out in 1969 and in three short campaigns between 1974 and 1989.
The site is very close to the ancient beach. The oldest walls were built in the second century. There are restorations from the second half of the fourth century (opus vittatum). On the east side baths were found. On the west side are rooms surrounding a trapezoidal courtyard with a porticus. To the north is a huge cistern with two big, internal piers. Amongst the rooms is a large rectangular hall with piers set against the long walls, supporting the ceiling. In the short back wall are four semicircular and rectangular niches for small statues. The room was entered through a monumental porch flanked by two columns. A small room on a corner of the courtyard has a small apse in the back wall, inside which is a brick podium that could have supported a statue of Isis (see below). There is furthermore a latrine with several seats.
In the rooms were found: a statue of a female deity, possibly Isis Pelagia or Pharia (h. 2.09); a statue of a bearded snake; a portrait of Septimius Severus as Serapis; another portret of this Emperor had already been found in this area in 1941; a herm of alabaster. The statue of, presumably, Isis is made of dark marble called bigio dorato in Italian, a marble that was often used for statues of Egyptian deities. The head, arms and feet are missing. They were probably made of white marble. The goddess is depicted in strong forward movement. She may have been standing on the prow of a ship with her left leg. The statue has been dated to the second half of the second century AD. The various rooms may have formed part of the seat of a guild related to the cult of Isis, and her temple may have been nearby.
The structures as seen in Google Earth. South is up for correct lightfall.