Which Guitar Hero game first included online multiplayer?

Guitar Hero III: Legends of Rock was the first Guitar Hero game to include online multiplayer. The feature was available in the PlayStation 3, Wii, and Xbox 360 versions of the game.

Earlier entries supported local multiplayer modes, including cooperative and competitive play, but they did not provide online multiplayer. Guitar Hero II's Xbox 360 release was discussed as a possible future platform for online play, yet the feature was not available when that version launched.

Guitar Hero III expanded the series beyond local living-room competition by allowing players to challenge or cooperate with other players over the internet. Its online functionality was paired with downloadable song support on the PlayStation 3 and Xbox 360 versions.

Later games, including Guitar Hero World Tour and Guitar Hero Live, continued or expanded the series' multiplayer features. They therefore cannot be the first entry, regardless of which platform version is being considered.