A fourth season of excavation was undertaken at Portus between late March and early May 2011. The work was directed by Simon Keay (BSR/University of Southampton), with Graeme Earl (University of Southampton) and Fabrizio Felici (Parsifal Cooperativa, Rome), with the assistance of a team of archaeologists from the University of Southampton and the BSR, and professional Italian archaeologists. The area chosen for excavation was Building 5, which lies immediately to the southeast of the Palazzo Imperiale in the centre of the port. Earlier work in 2008 and 2009 had cleaned the external face of the building that had been incorporated into the Mura Costantiniane in the later fifth century AD, as well as uncovering the northern end of what appeared to have been a corridor and stairs. These were interpreted as belonging to a single massive building of uncertain function that extended for c. 195 m from west to east between the main body of the Palazzo Imperiale and the eastern limit of this side of the Trajanic harbour basin. The 2011 excavations indicate instead that the structures excavated in 2008 and 2009 belonged to the north–south façade of a structure that extended c. 60 m westwards (Building 5) towards the main bulk of the Palazzo Imperiale. It comprised a series of long corridors and originally would have had at least two storeys and stood to c. 20 m. Building 5 was bordered to the east by a wide, and apparently open, area that would have facilitated communication between the Trajanic and Claudian basins. There seems little doubt that this structure would have been an integral part of the Palazzo Imperiale, forming its southeastern quadrant. Indeed, its façade is perfectly aligned with the eastern face of Building 1 to the north. In chronological terms it was of Trajanic date.

The open area bordering Building 5 to the east was defined on its eastern side by what has now been identified as the true western side of Building 7, which extends eastwards for 165 m. This consisted of long broad bays c. 12 m wide and 58 m long that ran from west to east the whole length of the building. Although the function of this building remains uncertain at the present, its primary façade and associated entrances lay along its northern façade, thus opening onto an extensive space c. 30 m wide, beyond which lay the east–west mole of the Claudian basin. This building also dated to the Trajanic period, and probably formed part of a broader complex of buildings that was focused upon the Palazzo Imperiale. The excavations also revealed good evidence for significant structural changes to Building 5, probably in the Severan period, a development that needs to be understood in conjunction with the transformation of the rest of the Palazzo Imperiale and, possibly, with the structural unification of this complex and the Grandi Magazzini di Settimio Severo.

As in other parts of the Palazzo Imperiale, the late antique period witnessed a major transformation in the function of both buildings. At some stage, the wide bay of Building 7 seems to have been sub-divided into a number of smaller units of uncertain function. Furthermore, the construction of the Mura Costantiniane in the later fifth century AD saw the masking of the façade and of the openings of Building 7, and its physical union with Building 5, reducing the gap between both structures to a small gateway. This was eventually blocked in the earlier sixth century as part of a broader project to reinforce the defences. In this, Buildings 7 and part of 5 were demolished systematically, in such a way as to enable the standing remains to form a substantial northward-facing rampart that sloped gently down to the Trajanic basin to the south. Subsequent to this, burials proliferated across the whole excavated area, as in other part of the Palazzo Imperiale.