FOSSOMBRONE (anc. Forum Sempronii), a town and episcopal see of the Marches, Italy, in the province of Pesaro and Urbino, 11 m. E.S.E. of the latter by road, 394 ft. above sea-level. Pop. (1901) town, 7531, commune, 10,847. The town is situated in the valley of the Metauro, in the centre of fine scenery, at the meeting-point of roads to Fano, to the Furlo pass and Fossato di Vico (the ancient Via Flaminia), to Urbino and to Sinigaglia, the last crossing the river by a fine bridge. The cathedral, rebuilt in 1772–1784, contains the chief work of the sculptor Domenico Rosselli of Rovezzano, a richly sculptured ancona of 1480. S. Francesco has a lunette by him over the portal. The library, founded by a nephew of Cardinal Passionei, contains some antiquities. Above the town is a medieval castle. There is a considerable trade in silk.
The ancient Forum Sempronii lay about 2 m. to the N.E. at S. Martino al Piano, where remains still exist. It was a station on the Via Flaminia and a municipium. The date of its foundation is not known. Excavations in 1879–1880 led to the discovery of a house and of other buildings on the ancient road (A. Vernarecci in Notizie degli scavi, 1880, 458). It already had a bishop in the years 499–502. In 1295 the Malatesta obtained possession of it, and kept it until 1444, when it was sold, with Pesaro, to Federico di Montefeltro of Urbino, and with the latter it passed to the papacy under Urban VIII. in 1631.