Susa (anc. Segusio, q.v.), a city and episcopal see of Piedmont, Italy, in the province of Turin, from which it is 33 m. W. by rail. Pop. (1901), 3607 (town); 5023 (commune). It is situated on the Dora Riparia, a tributary of the Po, 1625 ft. above sea-level, and is protected from the northern winds by the Rocciamelone. Among the medieval buildings of Susa the first place belongs to the church of San Giusto, founded in 1029 by Olderico Manfredi II. and the countess Berta, and in 1772 raised to be the cathedral. It has a fine brick campanile and brick decoration, and contains a bronze triptych of 1358 in niello, with the Virgin and Child. In the Valle di Susa, about 14 m. east of it, towards Turin, near S. Ambrogio di Torino, is the monastery of S. Michele with a Romanesque church, situated on a rocky mountain (998-1002).
After the time of Charlemagne a marquisate of Susa was established; and the town became in the 11th century the capital of Adelaide countess of Savoy, who was mistress of the whole of Piedmont. On his retreat from Legnano in 1176 Barbarossa set fire to Susa; but the town became more than ever important when Emmanuel Philibert'fortified it at great expense in the 16th century. It was, however, dismantled by Napoleon I. in 1796.