The longest river in Spain is the Tagus, also called the Tajo. About 445 miles of its 645 miles run through Spain, with the rest running through Portugal and into the Atlantic Ocean in Lisbon.
The Tagus is also the longest river in the entire Iberian Peninsula. It originates in high elevations east of Madrid, and passes through Teruel and Toledo as it flows southwest toward Lisbon. The Guadarrama River joins the Tagus' flow in Toledo, the largest Spanish city along the river. Many of the regions along the Tagus are difficult to establish large settlements upon, due to steep and rocky terrain.