Rantasalmi is a municipality in Finland, located in the South Savo region. It lies 43 kilometres (27 mi) northwest of Savonlinna and 92 kilometres (57 mi) northeast of Mikkeli.
The municipality has a population of 3,219 and covers an area of 925.20 square kilometres (357.22 sq mi), of which 366.61 square kilometres (141.55 sq mi) is water. Its population density is 5.75 inhabitants per square kilometre (14.9/sq mi).