Mértola (Portuguese pronunciation: [ˈmɛɾtulɐ]) is a municipality in southeastern Portugal next to the Spanish border. It has a total area of 1,292.87 km² and a total population of 8,712 inhabitants (2001). In terms of land area, it is the sixth-largest municipality in Portugal. The resultant population density of 6.74 persons/km² is the second-lowest in Portugal, behind only that of adjacent Alcoutim. The municipality is composed of 9 parishes and is located in the southeastern part of the District of Beja, in the Lower Alentejo region.

The seat of the municipality is the town of Mértola, which has around 3100 inhabitants. The town of Mértola is located on a hill by the Guadiana river, and its strategic location made it an important fluvial commercial port from Classical Antiquity through the period of Islamic domination. Among the vestiges of its past, Mértola boasts the only mediaeval mosque to have survived in Portugal.

The present Mayor is Jorge Rosa, elected by the Socialist Party.