The church of St Mary's stands within the walls of Portchester Castle and was built for the Augustinian priory soon after 1130. It proved to be an unsuitable site and the canons moved to Southwick c.1147-50. There is little surviving evidence of the monastic buildings, only a scar where the west range joined the church and the lavatories of the reredorter on the south fortress wall.
It is an outstanding Romanesque church of cruciform plan and consisted of a nave, chancel, transepts, central tower, and north chapel. The chancel has been shortened and the south transept demolished, but apart from this, the church remains virtually intact.
Externally the church is fairly plain with simple round-headed windows, the exception being the west end which is splendidly decorated. The west door arch has several bands of zigzag mouldings and sexfoil florets within circles. Two of the roundels have signs of the zodiac, Pisces and Sagittarius. The shafts have incised spiral and V decoration, with some volute capitals. Above the door the central window is flanked by two smaller blank arches infilled with a pattern of quatrefoils within circles. All three arches have zig-zag surrounds.
The interior decoration is reserved mainly for the tower crossing. The arches have mouldings of billets, and simple capitals of scallops and fluted stem patterns, others have elaborate volutes on the angles with foliated patterns on the undersides.
Only the arches of the arcading that once decorated the chancel walls remain. This theme is continued in the north transept where the arcades are complete, having shafts and highly decorated capitals. These are considered to be replacements, but some on the north wall look original.
The beautifully decorated font, probably 12th century, harks back in design to an earlier period. It has a lower pattern of intersecting arches, with an inhabited vine scroll above.
Pevsner, N., and Lloyd, D., 1967. The Buildings of England. Hampshire and the Isle of Wight (London, Penguin), 382-6