Tagus (Spanish, Tajo; Port., Tejo), river, the longest of the Iberian Peninsula. It rises in the Sierra de Albarracín in eastern Spain and flows generally west for 1,007 km (626 mi) to enter the Atlantic Ocean at Lisbon, Portugal. Only the lower course of the river is navigable. In its central course, around Toledo, Spain, the river's waters are used to irrigate fertile farmlands. Both the upper and lower-middle courses of the river flow through steep gorges. Numerous dams exploit the river's great hydroelectric potential, and there are several large reservoirs. The river's mouth is the site of an extensive port around Lisbon.