The interior lake district is the largest geographic region, and it is perhaps what most foreigners think of when they imagine Finland. The district is bounded to the south by the Salpausselka Ridges. Behind the ridges extend networks of thousands of lakes separated by hilly forested countryside. This landscape continues to the east and extends into the Soviet Union. As a consequence, there is no natural border between the two countries. Because no set definition of what constitutes a lake and no procedures for counting the number of lakes exist, it has been impossible to ascertain exactly how many lakes the region has. There are, however, at least 55,000 lakes that are 200 or more meters wide. The largest is Lake Saimaa, which, with a surface area of more than 4,400 square kilometers, is the fifth largest lake in Europe. The deepest lake has a depth of only 100 meters; the depth of the average lake is 7 meters. Because they are shallow, these many lakes contain only slightly more water than Finland's annual rainfall. The hilly, forest-covered landscape of the lake plateau is dominated by drumlins and by long sinuous eskers, both glacial remnants.

Upland Finland extends beyond the Arctic Circle. The extreme north of this region is known as Lapland. The highest points in upland Finland reach an elevation of about 1,000 meters, and they are found in the Kilpisjarvi area of the Scandinavian Keel Ridge. In the southern upland region the hills are undulating, while in the north they are rugged. Much of upland Finland is not mountainous, but consists of bogs.

Finland's longest and most impressive rivers are in the north. The Kemijoki has the largest network of tributaries. Farther south the Oulujoki drains the beginning of the north country. Most of the streams flow to the Gulf of Bothnia, but there is a broad stretch of land in the north and northeast that is drained by rivers flowing north across Norway and northeast across the Soviet Union to the Arctic Ocean.