One of three planets in the Japreal System, Onderon has a temperate climate, and was once home to a primitive race of humanoids. Like many other fringe worlds, the Onderonians have a distrust of aliens.

It has four moons. Dxun, the primary moon, once orbited very close to the planet, allowing the various Dxun beasts to move between the planet and the moon.

The native Onderonians struggled against the beasts, eventually gathering new technologies and building the huge walled city (and only city on Onderon) of Iziz to protect themselves from the beasts. Later, the Onderonians began to send their criminal element out into the wilderness without any protection from the beasts. These outcasts soon learnt to defend themselves, and eventually formed a second civilization. Relations between the two groups were hostile, and they remained at war with each other for many years.

Eventually the Old Republic asked the Jedi to watch over the world. The Jedi Master Arca Jeth took over as watchman and assigned his padawan Ulic Qel-Droma to the planet. The war was finally ended when the Onderonian princess Galia married the Beastprince - although tensions remained for long after that.

Many years earlier, Onderon was also the planet on which the Sith Freedon Nadd attempted to found a new Sith Empire. As a matter of fact, he founded the Onderon royal line and his tomb is located on the moon of Dxun. It is also rumoured that Darth Bane went to Onderon centuries later to find an apprentice.

Onderon is featured in the game Star Wars: Knights of the Old Republic II: The Sith Lords.