The Tagus (Latin Tagus, Spanish Tajo, Portuguese Tejo, pron. 'tɛʒʊ, Ancient greek Tàghos, Ταγος) is the longest river on the Iberian Peninsula. It measures 1,038 kilometers in length, 716 km of which are in Spain, 47 km as border between Portugal and Spain and the remaining 275 km in Portugal, where it empties into the Atlantic at Lisbon. It drains an area of 80,100 km² (the second largest in the Iberian peninsula after the Douro). It follows a very constricted course for much of its length, but after Almourol it enters a vast alluvial valley prone to flooding. Today the Alcantara Dam regulates much of the river's flow.
The source of the Tagus is the Fuente de García, in the Albarracín mountains. All its major tributaries enter the Tagus from the right (north) bank. The main cities it passes through are Aranjuez, Toledo and Talavera de la Reina in Spain, and Abrantes, Santarém, Almada and Lisbon in Portugal.
After the stretch where the Tagus forms the border, it enters Portugal, passing by Vila Velha de Ródão, Abrantes, Constância, Entroncamento, Santarém and Vila Franca de Xira at the head of the long narrow estuary, which has Lisbon at its mouth. There is the largest bridge across the river, the Vasco da Gama Bridge (in Lisbon) with a total length of 17.2 km. It is also the longest bridge in Europe. The Pepper Wreck is the name of a shipwreck located and excavated at the mouth of the Tagus River between 1996 and 2001.
The Portuguese regions of Alentejo and Ribatejo take their names from the river. Alentejo, from Além-Tejo ("beyond the Tagus"), and Ribatejo, from Arriba-Tejo (on the banks of the Tagus). There is a canal and aqueduct between the Tagus and the river.