Mäntyharju 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 7,013 (2003) and covers an area of 1,210.49 km² of which 228.03 km² is water. The population density is 7.1 inhabitants per km².

Mäntyharju is a popular holidayplace, mostly because of its pure lakes.

The municipality is unilingually Finnish.