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 500 feet (150 m) wide in most locations yet, at 33 miles (53 km), claims to have the longest "Main Street" in the world.
Island Park is part of the Rexburg Micropolitan Statistical Area.
Island Park is located at 44°29′59″N 111°20′19″W (44.4996, −111.3387), at an elevation of 6,293 feet (1,918 m) above sea level, making it the highest city in Idaho.
According to the United States Census Bureau, the city has a total area of 6.77 square miles (17.53 km2), of which, 6.64 square miles (17.20 km2) is land and 0.13 square miles (0.34 km2) is water.
The area was known as Island Park long before the 33-mile (53 km)-long town was incorporated. The area known as Island Park is mostly a large crater or caldera named the Henry's Fork Caldera that was created by the same hotspot that created the earlier Island Park Caldera and the later Yellowstone Caldera.
In addition to the portion which lies on the Henry's Fork Caldera, about a third of what is known as Island Park is north of the caldera, extending across Henry's Lake Flat and Henry's Lake towards the Idaho/Montana border. Henry's Lake Flat is a flat bottomed valley between high mountain ranges, with Henrys Lake at the northwest end of the flats. Mount Jefferson, south of Henrys Lake, is 10,203 feet (3,110 m) high and Targhee Peak, north of Henrys Lake is 10,240 feet (3,121 m) high. The most famous of Island Park's peaks, however, is the 9,886-foot (3,013 m) Sawtell Peak, south of Henrys Lake near Mount Jefferson.