Saimaa is a lake in southeastern Finland. At approximately 4,400 square kilometres (1,700 sq mi), it is the largest lake in Finland, and the fourth largest natural freshwater lake in Europe. It was formed by glacial melting at the end of the Ice Age. Major towns on the lakeshore include Lappeenranta, Imatra, Savonlinna, Mikkeli, Varkaus, and Joensuu. The Vuoksi River flows from Saimaa to Lake Ladoga. Most of the lake is spotted with islands, and narrow canals divide the lake in many parts, each having their own names (major basins include Suur-Saimaa, Orivesi, Puruvesi, Haukivesi, Yövesi, Pihlajavesi, and Pyhäselkä). Thus, Saimaa exhibits all major types of lake in Finland at different levels of eutropification.

Hietasaari island
In places in the Saimaa basin (an area larger than the lake), "there is more shoreline here per unit of area than anywhere else in the world, the total length being nearly 15,000 kilometres (9,300 mi). The number of islands in the region, 14,000, also shows what a maze of detail the system is."

The Saimaa Canal from Lappeenranta to Vyborg connects Saimaa to the Gulf of Finland. Other canals connect Saimaa to smaller lakes in Eastern Finland and form a network of waterways. These waterways are mainly used to transport wood, minerals, metals, pulp and other cargo, but also tourists use the waterways.

An endangered freshwater seal, the Saimaa Ringed Seal, lives only at Saimaa. Another of the lake's endangered species is the Saimaa salmon.

About 6000 year ago, ancient Lake Saimaa, estimated to cover nearly 9000 km2 at the time, was abruptly discharged through a new outlet. The event created thousands of square kilometres of new residual wetlands.Following this event, the region saw a population maximum in the decades following only to later return to an ecological development towards old boreal conifer forests which saw a decline in population.

Due to its rich, easily accessible asbestos deposits, the shores of the lake are the most probable origin of asbestos-ceramic, a type of pottery made between c. 1900 BC – 200 AD.