Mjøsa is Norway's largest lake. It is located in the southern part of Norway, about 100 km north of Oslo. Its main tributary is Gudbrandsdalslågen in the north; the main distributary is Vorma in the south.

From its southernmost point at Minnesund in Eidsvoll to its northernmost point in Lillehammer it is 117 km long. At its widest, near Hamar, it is 15 km wide. It is 365 km² in area, its volume is estimated at 56 km³, normally its surface is 123 meters above sea level, and its greatest depth is 449 meters. Its total coastline is estimated at 273 km, of which 30% is built up. Dams built on the distribuary of Vorma in 1858, 1911, 1947, and 1965 raised the level by approximately 3.6 meters in total. In the last 200 years, 20 floods have been registered that added 7 meters to the level of Mjøsa. Several of these floods inundated the city of Hamar.

The cities of Hamar, Gjøvik, and Lillehammer were founded along the shores of the lake. Aside from minor leisure boating and the steamship Skibladner, there is no water traffic on the lake. Most of its shores are dominated by rolling agricultural areas, among them some of the most fertile grainlands in Norway. The main train line between Oslo and Trondheim goes along its eastern shore, making stops in Hamar and Lillehammer. The largest island is Helgøya .