Ruotsinsalmi sea fortress is a fortification system in Kotka, Finland. It is part of the South-Eastern Finland fortification system built by Russia after Russo-Swedish War of 1788-1790. Ruotsinsalmi sea fortress formed the southern part of a double fortress together with Kyminlinna and it was built to counter the Swedish sea fortresses of Svartholm in Loviisa and Sveaborg (Suomenlinna) in Helsinki. Ruotsinsalmi also acted as an outpost of the Kronstadt sea fortress in Saint Petersburg. During the Crimean War, a British-French fleet destroyed the Ruotsinsalmi fortifications in 1855.