Hvaler is an island municipality located at the south-eastern end of the Oslofjord, directly south of Fredrikstad. The municipality consists of 833 islands, islets, and skerries at least 20 square meters in size. The largest town is Skjærhalden, situated in the southern part of Hvaler on the island of Kirkøy.
Ytre Hvaler National Park is primarily a marine park, stretching from the southern coast of the Hvaler islands out into the sea. The national park was created to preserve the relatively untouched coastal nature as well as the animal species that call the area home.
Another goal of establishing the national park was to maintain the varied underwater landscape, from the shallow and exposed areas at Heia, to the 460-meter deep seabed between Søsterøyene and Torbjørnskjær. The underwater landscape in this area has a diverse selection of plants and is also home to many different coral reefs. The 1,200-meter-long Tisler reef is particularly special.
Four former seabird reserves, Akerøya, Heia, Møren, and Søndre Søster, are now part of Ytre Hvaler National Park.
Ytre Hvaler National Park borders Færder National Park to the west and to the south lies the Swedish Kosterhavnet National Park, which was established at the same time.