Rensselaer is a city located along the Iroquois River in Marion Township, Jasper County, Indiana, United States. The population was 5,859 at the 2010 census. The city is the county seat of Jasper County. Rensselaer is home to Saint Joseph's College of Indiana.
Rensselaer is located at 40°56′17″N 87°9′5″W / 40.93806°N 87.15139°W / 40.93806; -87.15139 (40.938051, -87.151341). U.S. Route 231 and Indiana State Road 114 intersect in the downtown area. The Iroquois River flows through the south part of the city.
According to the United States Census Bureau, the city has a total area of 3.0 square miles (7.7 km²), of which, 2.9 square miles (7.5 km²) of it is land and 0.1 square miles (0.2 km²) of it (2.03%) is water.