Island Park is a city in Fremont County, Idaho, United States. The city's population was 286 at the 2010 census, up from 215 in 2000. The city was incorporated by owners of the many lodges and resorts along U.S. Route 20 in 1947, primarily to circumvent Idaho's liquor laws that prohibited the sale of liquor outside of city limits. It is only wide in most locations and, at , claims to have the longest "Main Street" in the world.