View over Tejo River from
São Jorge Castle in Lisbon (June 2002).

Tagus (Latin Tagus, Spanish Tajo, Portuguese Tejo) is the largest 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.

The source of the Tagus is the Fuente de García , in the Albarracín mountains. It ends in the Atlantic Ocean by Lisbon.

Its delta is partially inverted, and forms a large lagoon.