Vättern is the second largest lake by surface area in Sweden, after Vänern, and the sixth largest lake in Europe. It is a long, finger-shaped body of fresh water in south central Sweden, to the southeast of Vänern, pointing at the tip of Scandinavia. Being a deep lake at 128 metres (420 ft) below sea level at its deepest point, Vättern is about 1/3 the surface area of Vänern but in spite of this contains roughly 1/2 of its water.

Vänern is the largest lake in Sweden, the largest lake in the European Union and the third-largest lake in Europe after Ladoga and Onega in Russia. It is located in the provinces of Västergötland, Dalsland, and Värmland in the southwest of the country. With its surface located at 44 metres (144 ft) with a maximum depth of 106 metres (348 ft), the lowest point of the Vänern basin is at 62 metres (203 ft) below sea level. The average depth is at a more modest 28 metres (92 ft), which means that the average point of the lake floor remains above sea level.