Luumäki is a municipality of Finland. Its seat is in Taavetti.

It is located in the province of Southern Finland and is part of the South Karelia region. The municipality has a population of 4,903 (30 June 2015) and covers an area of 859.83 square kilometres (331.98 sq mi) of which 109.77 km (42.38 sq mi) is water. The population density is 6.54 inhabitants per square kilometre (16.9/sq mi). Neighbour municipalities are Hamina, Kouvola, Lappeenranta, Lemi, Miehikkälä and Savitaipale.