The Caia (Portuguese pronunciation: [ˈkajɐ]) is a river in the Iberian Peninsula, a tributary to the Guadiana.

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.

It has its sources in the Serra de São Mamede and for the lower 11 kilometres (7 mi) of its course it forms the international Portugal-Spain border.

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