Suomenlinna, Sea fortress on islands near Helsinki, Finland.

Suomenlinna is a sea fortress spread over eight islands about four kilometres southeast of Helsinki's centre, six of which are fortified. Its dark, weathered stone walls and low rectangular buildings with dark pitched roofs follow the uneven rocky terrain. On the central island, a large oval area is enclosed by earthen ramparts and stone walls, and bridges link the main islands over the surrounding water.

Sweden began building the fortress in 1748 under Admiral Augustin Ehrensvärd, following Vauban's bastion principles adapted to rocky islands. Russia besieged it in 1808 and it surrendered on 3 May, leading to Russian rule over Finland in 1809. After Finnish independence in 1917, it was renamed Suomenlinna in 1918 and served as a prison camp for Red soldiers after the Civil War.

The Ehrensvärd Museum occupies the commandant's house on the Great Courtyard, where Augustin Ehrensvärd is buried; King Gustav III personally sealed the tomb's granite slab on 5 July 1783. The Suomenlinna Church, built 1850–1854 in Orthodox style with onion domes, became Lutheran in the 1920s and carries a lighthouse flashing Morse H for Helsinki. The submarine Vesikko, completed in 1933 and serving in the Winter and Continuation Wars, sits at Susisaari's southeastern tip.

Ferries run year round from Helsinki's Market Square to the main pier, a crossing of about 15 minutes. Five of the eight islands are linked by bridges or a sandbar, so once ashore you can walk between them. Addresses use an island letter code plus a house number, such as C 83 on Iso Mustasaari.

The fortress has its own postal code, 00190, and addresses use an island letter plus a house number rather than a street name, so a letter might read "C 83" for a building on Iso Mustasaari. The district counts over 600 residents, and a minimum-security prison founded in 1971 holds about 100 inmates whose work is used in repairing the walls and ramparts.