Ad Pirum was a Roman fortress active during the time of the late Roman Empire. It is located on the Hrušica Plateau in southwestern Slovenia, in the hamlet of Hrušica in Podkraj. It was built in the 320s.
Excavations show the fortress to be of oval shape, 250 metres (820 ft) in length, 30 to 80 m (98 to 262 ft) in width, and with a wall at a height of 6–8 m and a thickness of about 2.7 m; the wall towers were 10 m high. This structure represented a major fort of the Claustra Alpium Iuliarum, a network of forts and walls to securing Italia from eastern invasions, including the road between Italia and Pannonia. Typically manned with 500 soldiers, the fortress could accommodate up to 100,000 soldiers.