Ta'gus, a river of the Iberian peninsula, rising northeast of the center of Spain and flowing southwesterly, then westerly, into Portugal, thence in a southwesterly course to the Atlantic. Its entire length is about 550 miles. The important tributaries all join it from the north. The most important are the Jarama, the Alberche and the Alagon. Most of the river's course is through an arid country, and its current is rapid.