38°52′32.1″N 7°2′10″W / 38.875583°N 7.03611°W
The Caia (Portuguese pronunciation: [ˈkajɐ]) or Caya 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 part of the border along the Guadiana between the rivers Caia and Ribeira de Cuncos, since the annexation of Olivenza by Spain in 1801. This territory, though de facto and de jure Spanish, remains de jure disputed by Portugal.