Luumäki is a municipality of Finland. Its seat is in the Taavetti village. It is located in the province of Southern Finland and is part of the South Karelia region. The municipality has a population of 4,379 (31 October 2024) and covers an area of 859.83 square kilometres (331.98 sq mi) of which 109.77 km2 (42.38 sq mi) is water. The population density is 5.84 inhabitants per square kilometre (15.1/sq mi). The municipality is unilingually Finnish.