We studied the breeding biology of the Japanese Night Heron Gorsachius goisagi from 21 June to 2 August 2009. This species was previously considered an endangered endemic breeding species of southern Japan only, but was found breeding in a valley at the bottom of Mt. Halla at Ara-dong, Jeju City, the southernmost island of Korea. This marks the first record of this species ever to be found breeding in Korea. The forested area consisted predominantly of plants such as Morus bombycis, Cryptomeria japonica, Styrax japonica, Orixa japonica, Viburnum dilatatum, and Lindera erythrocarpa. Materials for the nest were largely M. bombycis, followed by S. japonica, C. japonica, and so forth. There were three eggs in the clutch, and the brooding period lasted for 42 days. A close look into the pellets demonstrated that their main prey items were earthworms, snails, and cicadas. The results of this study represent valuable data, allowing for the mapping out of new methods for managing Gorsachius goisagis, a species currently on the brink of extinction.