lake in south-central Sweden, southeast of Lake Väner (Väner, Lake) between the administrative län (counties) of Västra Götaland and Östergötland and north of the traditional landskap (province) of Småland. With a length of 81 miles (130 km), a breadth of about 19 miles (31 km), and an area of 738 square miles (1,912 square km), it is Sweden's second largest lake, though only one-third the size of Lake Väner. It has a maximum depth of 420 feet (130 metres) and a surface 289 feet (89 metres) above sea level. The lake, known for its dangerous currents, drains eastward through the Motala River into the Baltic Sea.

Lake Vätter is bounded by cliffs to the east and west; there are few harbours, and Vising Island (Visingsö), with an area of 9.5 square miles (24.5 square km), is one of the few islands. The region around the lake developed after 1832 with the opening of the Göta Canal, which uses the lake and continues on to Stockholm at Motala, on the northeastern shore. Jönköping, at the southern end, is the largest town on the lake. Tourism is important at Vadstena, on the eastern shore south of Motala, with St. Bridget's Convent (c. 1383); Kloster Kyrkan (Convent Church; 1395–1424), also known as the Blue Church from its bluish-gray limestone; and the 16th-century castle of King Gustav I Vasa. On the western shore, Hjo developed as a spa in the late 18th century and still thrives as a lakeside resort.