Vänern ( VAYN-ərn, also US: VEN-, Swedish: [ˈvɛ̂ːnɛɳ]) is the largest lake in Sweden, the largest lake in the European Union and the third-largest lake of all 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.

Vänern drains into Göta älv towards Gothenburg and the Kattegat tributary of the Atlantic Ocean. It is the only one of the ten largest lakes in Sweden not to drain on the country's eastern coastline. Due to the construction of Göta Canal in the 19th century, there is an upstream water path to Vättern and the east coast from Vänern. The main inflow of water comes from Klarälven entering Vänern near Karlstad with its source in Trøndelag in Norway.