The Mirna is a river in southeastern Slovenia. The river, in which the marathon swimmer Martin Strel first learned to swim, is a right tributary of the Sava River in the province of Lower Carniola. It is long, starts below the settlement of Velika Preska, flows through the Mirna Valley and joins the Sava at Dolenji Boštanj, opposite Sevnica. The largest settlement on the river is Mirna. The river is traversed by the Sevnica–Trebnje Railway.