Does the Tagus River have its source or its mouth in Spain?

The source of the Tagus River is located in the Sierra de Albarracín mountains east of Madrid, Spain. It flows approximately 585 miles through Spain and into Portugal, reaching its mouth at the Atlantic Ocean near Lisbon, Portugal.

The Tagus flows in a generally southwest direction through Spain and forms 29 miles of the border between Spain and Portugal. The mouth of the Tagus River forms a large estuary and what many consider to be one of the best harbors in Europe. The Tagus River's name is derived from Latin. In Spain, this river is called the Tajo; in Portugal, it is called the Tejo.