The Treaty of Fredrikshamn (Freden i Fredrikshamn in Swedish and Haminan rauha in Finnish ) was a peace treaty concluded between Sweden and Russia on September 17, 1809. The treaty concluded the Finnish War and was signed in the present-day Finnish town of Hamina, which was located in a part of Finland that previously had been ceded to Russia by Sweden.

According to the treaty Sweden ceded Åland, parts of the provinces Laponia and Westrobothnia (east of the rivers of Tornio and Muonio) and all provinces east thereof.

The ceded territories came to constitute a Russian Grand Duchy, to which also the Russian 18th century conquests of Karelia including small parts of Nylandia and Savonia were joined in 1812 as Province of Viipuri, see "Old Finland" (i.e. old seen from a Russian perspective).

Together with the Porvoo Diet (1809), and the Oath of the Sovereign, the Treaty of Fredrikshamn constitutes the cornerstone for the autonomous Grand Duchy of Finland, its own administration and institutions, and thereby a start of the development which would lead to the revival of Finnish culture, to equal position of the Finnish language, and ultimately in 1917 to Finland's independence.

A reference to Tsar Alexander's promise to retain old laws and privileges in Finland was included, but the treaty overstepped any formal guarantees of the legal position of Finland's inhabitants. The Russians refused, and the Swedes were not in a position to insist. Similar clauses had been common in peace treaties, but they were also regularly circumvented. At the period of Russification of Finland, 90 years later, the Russian government argued that the treaty wasn't violated and hence no outside party had any right to intervene, the question being solely a matter of the Tsar who had granted the original promise.

Also for Sweden, the treaty turned out to be ultimately beneficial. Instead of the Åland islands, Sweden came to retain vast areas in the far North, already conquered by the Russians, where later important iron ore and hydropower were to constitute the basis for Sweden's rapid 20th century industrialization.

During the negotiations, Swedish representatives had namely endeavoured to escape the loss of the Åland islands, "the fore-posts of Stockholm," as Napoleon rightly described them. (The Åland islands were additionally culturally, ethnically and linguistically purely Swedish, but such facts were of no significance at that time.) In the course of the 19th century it would also turn out that the Åland islands were a British interest, which after the Crimean War led to the demilitarization of the islands according to the Åland Convention included in the Treaty of Paris (1856).