Suomenlinna (Finnish; until 1918 Viapori), or Sveaborg (Swedish), is an inhabited sea fortress built on eight islands about 4 km southeast of the city center of Helsinki, the capital of Finland. Suomenlinna is a UNESCO World Heritage site that is popular with tourists and locals, who enjoy it as a picturesque picnic site. 

Suomenlinna is now one of the most popular tourist attractions in Helsinki as well as a popular picnicking spot for the city’s inhabitants. On a sunny summer day the islands, and in particular the ferries, can get quite crowded. In 2009, a record 713,000 people visited Suomenlinna, most between May and September. A number of museums exist on the island, as well as the last surviving Finnish submarine, Vesikko.The M/S Suomenlinna II transports residents, tourists and vehicles to and from the harbor at Suomenlinna.

Suomenlinna has always been much more than just a part of Helsinki — it is a town within the town. There are about 900 permanent inhabitants on the islands, and 350 people work there year-round. This is one of the features that makes Suomenlinna unique: the fortress is not simply a museum but a living community.