| Caia (Rio Caia) | |
| Source1 Location: | Serra de São Mamede, Portugal |
| Mouth Location: | Guadiana River, near Badajoz, Spain |
| Subdivision Type1: | Country |
| Subdivision Name1: | Spain, Portugal |
| Basin Size Km2: | 846 |
The Caia (pronounced as /pt/) 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.
It has its sources in the Serra de São Mamede and for the lower 11km (07miles) of its course it forms the international Portugal-Spain border. Finally it joins the Guadiana River southwest of the city of Badajoz.