Mäntyharju (Finnish pronunciation: [ˈmæntyˌɦɑrju], "Pine Ridge") is a municipality of Finland.

It is located in the province of Eastern Finland and is part of the Southern Savonia region. The municipality has a population of 6,236 (30 June 2015) and covers an area of 1,210.99 square kilometres (467.57 sq mi) of which 229.23 km (88.51 sq mi) is water. The coastline is almost 1,520 kilometers (940 mi). The population density is 6.2 inhabitants per km².