Engetsu Island

 is a small island off the coast of Shirahama, Wakayama Prefecture, Japan. It includes a natural arch through which seawater passes. The hole in the middle of the island has been formed through the effects of wind and waves over time and has given it its nickname of Engetsu (full moon). Its official name is , and it is approximately east-to-west, north-to-south, and high at the tallest.
The island is largely formed of sandstone and has become less stable over time. Since July 2009, the Shirahama town government has cautioned against approaching the island because of the risk of collapse of the arch.