Seskar (Finnish: Seiskari) (Russian: Сескар) is an island in the Gulf of Finland, part of the Leningrad Oblast of Russia. The island was an independent municipality of Finland populated by Finns at least since 16th century, until 1940, when ownership was transferred to the Soviet Union following the Moscow Peace Treaty. The ownership was ratified after World War II.
The municipality Seiskari had been formed from the Koivisto municipality in 1903, and belonged to the province of Viipuri and to the South Karelia region.