Tiber (It. Tevere; anc. Tiberis), a river of Italy, rising in the Tuscan Apennines, 5 m. N. of Pieve San Stefano, and thence flowing generally S. S. E. and S. S. W., through the provinces of Arezzo and Perugia and between the latter and Viterbo, to Fiano, where it turns S. W., and passing through the province and city of Rome discharges into the Mediterranean near Ostia by two mouths, which enclose the Insula Sacra of the ancients. Its length is about 230 m., and its width at Rome and below from 300 to 500 ft. Its principal affluents are the Ohiascio, the Nera (anc. Nar), and the Teverone (Anio) from the left, and the Paglia from the right. In the upper part of its course, between Todi and the Passo del Forello, it is obstructed by rapids and passes for some miles through a narrow gorge. It is navigable for vessels of 130 to 200 tons to Rome, 18 m. from its mouth, and for boats to the confluence of the Nera, about 90 m. Rome and Perugia are the principal cities in its basin. From Perugia, above its confluence with the Chiascio, to its debouchure, its waters have a yellowish tinge, the result of the yellow clay through which it passes.

An appropriation was granted at the end of 1875 by the Italian government for the preliminary measures relating to the improvement of the course of the Tiber. Various plans are proposed for that purpose, including one of vast scope advocated by Garibaldi.