Tagus (Span. Tajo; Port, Tejo), a river of Spain and Portugal, the longest in the peninsula, and dividing it into two nearly equal parts. It rises in the Sierra de Cuenca, in the province of Teruel near the border of Guadalajara, and flows INT. W. for about 35 m.; then nearly W. 20 m., receiving the waters of the Molina; then S. W. a little more than 70 m., the Gua-diela and other streams augmenting it; then nearly W. till it becomes for about 20 m. the boundary of Portugal, its principal affluents being the Jarama, Cedron, Guadarrama, Al-berche, and Alagon. Entering Portugal, it inclines more and more to the southwest, receiving below Abrantes the Zezere, and from that point is navigable for vessels of 150 tons. In the lower part of its course are numerous islands, and for about 20 m. it spreads out into a table-like basin, 8 m. or more in width; but as it approaches Lisbon the hills on either sido close up the valley, and at its mouth it is not over a mile wide. The banks of the Tagus arc generally rugged and precipitous, and the adjacent plains are dry and barren. Lisbon, San-tarem, and Abrantes in Portugal, and Talavera de la Reyna, Toledo, and Aranjuez in Spain, are on its banks.

Its length is about 540 m.