Mértola is a town and a municipality in southeastern Portugal next to the Spanish border. The population in 2011 was 7,274, in an area of 1292.87 km². In terms of land area, it is the sixth-largest municipality in Portugal. The resultant population density of 5.62 persons/km² is the second-lowest in Portugal, behind only that of adjacent Alcoutim. The municipality is located in the southeastern part of Beja District, in the Lower Alentejo region.