According to the United States Census Bureau, the township has a total area of 28.4 square miles (73.5 km²); 24.3 square miles (63.0 km²) of it is land and 4.1 square miles (10.5 km²) of it (14.31%) is water.
Southside Township is located in Township 121 North of the Arkansas Base Line and Range 28 West of the 5th Principal Meridian.