The Caia is a river in the Iberian Peninsula, a tributary to the Guadiana. It is one of the main water courses in the Portalegre District, Portugal. Portugal does not recognise the border between the Caia and Ribeira de Cuncos River deltas, since the beginning of the 1801 occupation of Olivenza by Spain. This territory, though under de facto Spanish occupation, remains a de jure part of Portugal, consequently no border is henceforth recognised in this area.

Course

It has its sources in the Serra de São Mamede and for the lower 11 km of its course it forms the international Portugal-Spain border. Finally it joins the Guadiana River southwest of the city of Badajoz.

Tributaries

Dams