Jyu-Oh-Sei (Japanese: 獣王星, Hepburn: Jū Ō Sei, lit. "Planet of the Beast King") is a Japanese manga series written and illustrated by Natsumi Itsuki. The manga was first serialized in Hakusensha's monthly shōjo manga magazine, LaLa in December 1993. Several chapters of the series were serialized in Hakusensha's other bi-monthly shōjo manga magazine, Melody. The chapters released were then collected into five tankōbon volumes and later re-released into three kanzenban volumes. It was licensed and released in English in North America by Tokyopop, with translations by Yuya Otake and English adaptation by Clint Bickham.

An 11-episode anime adaptation was animated by Bones and premiered on April 13, 2006 in Japan as part of Fuji TV's Noitamina programming block. On May 2, 2009, the series made its North American television debut on the Funimation Channel.

The series deals with the adventures of twins Thor and Rai, who live on space colony Juno, a planet resembling Earth, until they are abandoned and sent to Chimaera, a secret planet dominated by extreme weather patterns which is used as a penal colony.