St. Anthony is a city in and the county seat of Fremont County, Idaho, United States. The population was 3,542 at the 2010 census, up fro…St. Anthony is a city in and the county seat of Fremont County, Idaho, United States. The population was 3,542 at the 2010 census, up from 3,342 in 2000. It is part of the Rexburg, Idaho Micropolitan Statistical Area.