Luumäki (Finnish pronunciation: [ˈluːˌmæki]) 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 4,327(30 June 2025) and covers an area of 859.83 square kilometres (331.98 sq mi) ofwhich 109.77 km (42.38 sq mi)is water. The population density is5.77 inhabitants per square kilometre (14.9/sq mi). The municipality is unilingually Finnish.
Neighbour municipalities are Hamina, Kouvola, Lappeenranta, Lemi, Miehikkälä and Savitaipale. Lappeenranta is located 38 kilometres (24 mi) from Luumäki.