Extertal is located on the northern edge of the circle in the Teutoburg Nature Reserve, directly adjacent to Lower Saxony. The Exter and Humme rivers flow through the region. The municipality has its headquarters in the Bösingfeld district. Neighbouring cities are Rinteln, Aerzen, Barntrup, Dörentrup and Kalletal.
The highest point of the city is at 371 meters above sea level. The municipality dimensions are about 12.2 kilometers east-west and about 12.5 kilometers north-south.