The Chao (チャオ) are a species from the Sonic the Hedgehog series. They are virtual pets that the player can take care of in some of in some Sonic games.
The Chaos are very similar to the Nightopians, as they not only bear many physical similarities to the Nightopians, but they also have a very similar Artificial life system. Much like the Nightopians, the Chaos can acquire physical traits of creatures who interact with them and their mood also changes depending on the player's behavior.
The Chaos have appeared in several crossovers featuring the NiGHTS series.
In the Sonic games which featured a Chao Garden, the player can make the Chaos look like NiGHTS and/or Reala by giving them flying-type animals. It is also possible to make them look like Jackle by giving them power-type animals,  however this one may be unintentional.
The NiGHTS Chao was made into an official plush toy as part of a Sonic Adventure set.
The Chaos appeared with the Nightopians as unlockable skins for the Chuchus. To unlock them, the player must beat all of the 50 courses from the game.
Sonic Pinball Party
The game has a Tiny Chao Garden has a side-feature where the player can take care of Chaos.
The Chaos appear in Sega Superstars, where there is a Tiny Chao Garden as a side-feature. In this game, a Chao can be hatched and petted via different movements. It is also possible to interact with some scenery (shaking trees for fruit) and purchase items for the Chao using rings collected in the main game mode.
Sega Superstars Tennis
The Chaos make a cameo in the Green Hill zone tennis court.
Sonic & All-Stars Racing Transformed
A Chao makes a cameo as a sticker.
- A Chao and a Nightopian appeared together as a capsule toy in the action-adventure game Shenmue for the Dreamcast.
- In some games, the Chaos were voiced by Tomoko Sasaki, the same person who voiced the Nightopians in NiGHTS: Journey of Dreams.