William Bay National Park is a national park in Western Australia, 369 km southeast of Perth.
Fifteen kilometres west of Denmark, William Bay National Park covers 1734 hectares and includes Greens Pool and Elephant Rocks. The granite boulders create a natural reef which protects Greens Pool from the Great Southern Ocean, and is a safe swimming beach for children (under supervision). William Bay National Park is located in the Great Southern Region of Western Australia along the Rainbow Coast, and is in the Shire of Denmark.
William Bay was named after the famed British Arctic explorer and navigator, Sir William Edward Parry, as were two other nearby features, Parry Inlet and Edward Point. The bay was named in the 1830s by John Septimus Roe.
The rare and ancient Main's assassin spider, currently listed as threatened, was found to inhabit the park during a survey conducted in 2008.