The siege of Sveaborg was a siege by Imperial Russian forces of the sea fort of Sveaborg, off the coast of Helsingfors (Helsinki); at the time Finland was part of the Kingdom of Sweden. It took place in the spring of 1808, during the Finnish War. Despite its formidable reputation as "the Gibraltar of the North", the fortress surrendered after a siege of two months. As its capitulation was followed by the rapid collapse of Swedish resistance elsewhere, and ultimately the Russian conquest of Finland, the siege is often regarded as the decisive battle of the war.

Suomenlinna, or Sveaborg, is an inhabited sea fortress composed of eight islands, of which six have been fortified; it is about 4 km southeast of the city center of Helsinki, the capital of Finland. Suomenlinna is popular with tourists and locals, who enjoy it as a picturesque picnic site. Originally named Sveaborg, or Viapori as referred to by Finnish-speaking Finns, it was renamed in Finnish to Suomenlinna in 1918 for patriotic and nationalistic reasons, though it is still known by its original name in Sweden and by Swedish-speaking Finns. Due to its strategic geographical location, it sometimes used to be known as Gibraltar of the North.