(Spanish, Tajo; Portuguese, Tejo), a river in Spain and Portugal. The longest river on the Iberian Peninsula, the Tagus is 1,010 km long and drains an area of 81,000 sq km. It originates in the Montes Universales and crosses the New Castile plateau from east to west and, in its lower course, the Portuguese lowland. It empties into the Atlantic Ocean, forming an estuary 45 km long. In the upper and middle courses, the valley is narrow and there are numerous rapids; in the lower course, the valley is wide and the river channel has caved in in places. The river is fed primarily by rain; there is high water from November through March and low water in summer. The mean flow rate at Alcántara, near the Spanish and Portuguese border, is 438 cu m per sec.
The Tagus and its tributaries are used for irrigation and hydroelectric power. There are several reservoirs with hydroelectric power plants on the river, including Buendía, Valdecañas de Tajo, and Alcántara in Spain. The river is navigable to Santarém (Portugal) for oceangoing vessels and to Abrantes (Portugal) for other vessels, a distance of 185 km from the mouth. The cities of Toledo (Spain) and Santarém are on the Tagus, and Portugal’s capital, Lisbon, is on the estuary.