Mäntyharju is a municipality in the South Savo region of Finland, located about 40 kilometres (25 mi) southwest of Mikkeli. It covers an area of 1,210.98 square kilometres (467.56 sq mi), of which 229.23 km² (88.51 sq mi) is water, and has a population of 7,034 (31 December 2025), giving a population density of 6.2 inhabitants per km². The municipality's coastline is almost 1,520 kilometres (940 mi) long.
The town lies on the Kouvola–Mikkeli railway line, and some trains originating as far away as Helsinki or Kajaani stop at Mäntyharju railway station. Bus services connect to Mikkeli, with the bus station in the town centre. Regional roads reach the municipality from five directions, including regional road 368 from highway 5 in the north and regional road 419 from highway 5 in the west. The Virransalmi Bridge is a local landmark.
A self-guided walking route, the Mäntyharju City Tour, runs 12.5 km from Mäntyharju Railway Station through the village centre to Uutela Shelter. It is waymarked with orange signs and arrows, and information maps along the way highlight places of natural beauty and cultural-historical interest; suggested stopping points include Pappilanniemi, Nuotioniemi shelter and Uutela shelter. An English-language brochure is available on the municipal website.