Rensselaer is a city located along the Iroquois River in Marion Township, Jasper County, Indiana, United States. The population was 5,733 at t…Rensselaer is a city located along the Iroquois River in Marion Township, Jasper County, Indiana, United States. The population was 5,733 at the 2020 census, down slightly from 5,859 at the 2010 census. The city is the county seat of Jasper County. Saint Joseph's College is located just south of the city limits.