The Tagus ( TAY-gəs; Spanish: Tajo [ˈtaxo]; Portuguese: Tejo [ˈtɛʒu]; see below) is the longest river in the Iberian Peninsula. The river rises in the Montes Universales near Teruel, in mid-eastern Spain, flows 1,007 km (626 mi), generally westward with two main south-west sections, and empties into the Atlantic Ocean in Lisbon. Its drainage basin covers 80,100 km² (30,927 sq mi) – exceeded in the peninsula only by the Douro. Several dams and diversions supply drinking water to population centers of central Spain and Portugal, and dozens of hydroelectric stations produce power. Between dams it follows a very constricted course, but after Almourol, Portugal it expands into a wide alluvial valley, with a prone to flooding. It culminates in a large estuary at the Port of Lisbon in the capital city of Portugal.

The source is specifically, in political geography, at the Fuente de García in the Frías de Albarracín municipality; in physical geography, in the Sistema Ibérico (Iberian System), covering the notably high mountains of the Sierra de Albarracín Comarca. The most plentiful tributaries (originating in the Central System mountain range) are on right bank, to the north. The river flows 716 km (445 mi) in Spain, 47 km (29 mi) along the two countries' borders and 275 km (171 mi) in Portugal.

The river flows through many cities, including Aranjuez, Toledo and Talavera de la Reina in Spain, and Abrantes, Santarém, Almada and Lisbon in Portugal. The Spanish capital, Madrid, lies on the right-bank side of the Tagus basin.