To the northwest of the ancient city of Ostia, the analysis of cores revealed a stratigraphic sequence, which we interpret as the filling of a harbour basin. This basin, located at the west of the so-called "Palazzo Imperiale" presents seven characteristics: (1) The maximum depth is 6 m below the Roman sea level. This depth allowed any type of ship (even heavy tonnage) to access the harbour. (2) A chronostratigraphic gap at −6 m below Roman sea level suggests digging operations in the basin (or subsequent dredging) that have caused the loss of sedimentary archives. (3) The filling consists of dark clays typical of a quiet environment but open to marine and river influences. (4) The dates at the base of this sequence give a range between the 4th and the 2nd century BC. (5) In the harbour sequence, a facies change at −2.5 m under the Roman sea level involves a change in the processes of sedimentation and/or operation. (6) No later than the beginning of the 1st century AD, the thickness of the water column in the basin is less than 50 cm and seems to be caused by a massive siltation following a succession of floods of the Tiber. (7) This basin was thus already abandoned during the start-up of Portus.