Greater Finland (Finnish: Suur-Suomi; Estonian: Suur-Soome; Swedish: Storfinland) is an irredentist and nationalist idea which aims for the territorial expansion of Finland. It is associated with Pan-Finnicism. The most common concept saw the country as defined by natural borders encompassing the territories inhabited by Finns and Karelians, ranging from the White Sea to Lake Onega and along the Svir River and Neva River—or, more modestly, the Sestra River—to the Gulf of Finland. Some extremist proponents also included the Kola Peninsula, Finnmark, Swedish Meänmaa, Ingria, and Estonia.

The idea of a Greater Finland rapidly gained popularity after Finland became independent in December 1917. The idea has lost support after World War II (1939–1945).

The concept of Greater Finland was commonly defined by what was seen as natural borders, which included the areas inhabited by Finns and Karelians. This ranged from the White Sea to Lake Onega and along the Svir River and Neva River. Alternatively, it ranged from the Sestra River to the Gulf of Finland. Some extremists also included the Kola Peninsula and Ingria in Russia, Finnmark in Norway, the Torne Valley in Sweden, and Estonia.

The idea of the so-called three-isthmus border—defined by the White Isthmus (between the White Sea and Lake Onega), the Olonets Isthmus (between Lake Onega and Lake Ladoga), and the Karelian Isthmus (between Lake Ladoga and the Gulf of Finland)—is hundreds of years old, dating back to the period when Finland was part of Sweden. There was a disagreement between Sweden and Russia as to where the border between the two countries should be. The Swedish government considered a three-isthmus border to be the easiest to defend.

Although the term "Greater Finland" was not used in the early 19th century, the idea of Finland's natural geographical boundaries dates back to then. In 1837, the botanist Johan Ernst Adhemar Wirzén defined Finland's wild plant distribution area as the eastern border lines of the White Sea, Lake Onega, and the River Svir. The geologist Wilhelm Ramsay defined the bedrock concept of Fennoscandia in 1898.

Karelianism was a national romantic hobby for artists, writers, and composers in which Karelian and Karelian-Finnish culture was used as a source of inspiration. Karelianism was most popular in the 1890s. For example, the author Ilmari Kianto, known as the "White friend", wrote about his travels to White Karelia in the 1918 book Finland at Its Largest: For the Liberation of White Karelia.

The Kvens, a minority in Northern Norway, helped Finnish settlements spread, especially in the 1860s. The Academic Karelia Society and the Finnish Heritage Association worked actively with the Kvens from 1927 to 1934, and the Finnish media spread pan-Fennicist propaganda through various channels. Activity slowed down from 1931 to 1934.

In the early days of its independence, Finland wanted Finnish-speaking areas in Norrbotten, Sweden, to join Finland. This was a reaction to the effort by Finland's own Åland to join Sweden. The Finnish government set up a committee to expand Finnish national movements. Sweden, for its part, pushed for instruction in the Swedish language in its northern Finnish regions. Until the 1950s, many schoolchildren in Norrbotten were banned from using the Finnish language during breaks at school.