Tonti Township (T3N R2E) is centered at 38°41'N 88°58'W (88.691,-88.975). It is transversed northeast-south by Interstate Route 57 and State Route 37. Part of the Salem Reservoir (El. 166 m) is located at the southeastern part of the township. According to the 2010 census, the township has a total area of 35.55 square miles (92.1 km2), of which 35.45 square miles (91.8 km2) (or 99.72%) is land and 0.1 square miles (0.26 km2) (or 0.28%) is water.