The Battle of Grengam (Russian: Гренгамское морское сражение, Swedish: Slaget vid Ledsund or Slaget vid Föglöfjärden[6]), Finnish: Flisön taistelu of 1720 was the last major naval battle in the Great Northern War that took place in Åland, in the Ledsund strait between the island communities of Föglö and Lemland.

The word Grengam in the name of the battle is a romanization of Гренгам, which is the traditional Russian cyrillization of Gränhamn, an older spelling of Granhamn.

The Swedish squadron consisting of a 52-gun ship of the line, four frigates and nine smaller craft with a total of 156 guns and over 1,000 marines, made an attempt to attack the moving Russian fleet.

General Mikhail Golitsyn managed to take an advantageous position in the narrow and shallow strait of Flisesund and ordered his ships into a semicircle formation.

[11] The Russian losses prevented their navy from launching any further major operations until the war ended with the Treaty of Nystad the following year.

No significant naval battles took place between the Russian and Swedish navies after this one until Sweden's defeat in the war was sealed by the Treaty of Nystad.

The Battle of Grengam demonstrated the skillful use of the rowing fleet in the skerries environment and efficient reconnaissance and selection of the combat site, as well as perfect timing for the attack from different directions.

[citation needed] The Swedish Vice Admiral Carl Georg Siöblad was initially criticised after the battle, but when the scope of the Russian losses were discovered he was praised.