Kostomuksha (Karelian: Kostamuš, Finnish: Kostamus) is a town in Russian Karelia.
Kostomuksha was built as a Finnish-Russian cooperation 1977–1985, for mining the iron ore found 1946, one of the largest iron ore reserves in Russia. The town functions also as a dacha-style resort (there is an abundance of lake shores in the area), and hosts a yearly summer chamber music festival. Most visitors are Finns.
Before 1920 the village preceding the town was an important source of folk poetry, gathered as part of the Finnish nation building from the 19th century onward. It was flooded as part of the industrialisation.
Kostomuksha has trains from Petrozavodsk and Saint Petersburg. The railway across the border to Finland serves only freight (especially iron for shipping via Finland).
The road A137 leads from the road R21 (Russian: Р-21 «Ко́ла», part of E105) through Kosomuksha to the border with Finland 30 km away, at Ljuttja (Люття)/Vartius near Kuhmo.
Primary administrative division