Coordinates: 60°30′N 29°54′E / 60.5°N 29.9°E / 60.5; 29.9

The Karelian Isthmus (Russian: Карельский перешеек) is the approximately 45–110 km wide stretch of land, situated between the Gulf of Finland and Lake Ladoga in northwestern Russia, to the north of the River Neva (between 61°21’N, 59°46’N and 27°42’E, 31°08’E). Its northwestern boundary is the relatively narrow area between the Bay of Vyborg and Lake Ladoga. If the Karelian Isthmus is defined as the entire territory of present-day Saint Petersburg and Leningrad Oblast to the north of the Neva, the isthmus' area covers about 15,000 km².

The smaller part of the isthmus to the southeast of the old Russia-Finland border is considered historically as Northern Ingria, rather than part of the Karelian Isthmus itself. The rest of the isthmus was historically a part of Finnish Karelia. This was conquered by the Russian Empire during the Great Northern War in 1712 and included within the autonomous Grand Duchy of Finland (1809–1917) of the Russian Empire. When Finland became independent in 1917, the isthmus (except for the territory roughly corresponding to present-day Vsevolozhsky District and some districts of Saint Petersburg) remained Finnish. Finnish Karelia was ceded to the Soviet Union by Finland following the Winter War (1939–1940) and Continuation War (1941–1944). In 1940–1941, during the Interim Peace, most of the ceded territories in the isthmus were included within the Karelo-Finnish SSR. However, since World War II the entire isthmus has been divided between the city of Saint Petersburg (mostly Kurortny District), as well as Priozersky District, Vsevolozhsky District and Vyborgsky District of Leningrad Oblast.