One of the most popular sights of Helsinki is the fortress of Suomenlinna (Sveaborg), where the museums of Suomenlinna and Ehrensvyard are located, a museum of toys and dolls, as well as galleries, parks and the Vesikko submarine. The fortress is listed as a UNESCO World Heritage Site.

Another important attraction of Helsinki is the Assumption Cathedral, a sample of Byzantine-Russian architecture, erected in 1868 and located on the island of Katajanokka. The Assumption Cathedral is considered to be the largest Orthodox church in Western Europe.

Of considerable interest is the Lutheran Church of Temppelinaukio, cut in the rock and covered with a glass dome, through which sunlight is pouring into its interior.