MU Online is a free-to-play isometric action MMORPG developed by the South Korean developer Webzen for Microsoft Windows. It is similar to Blizzard Entertainment's Diablo series.

"MU" refers to the mythical lost continent of Lemuria, also known as the Land of Mu, on which the game is set.

The MU IP has since released other games, made by various developers, such as MU Origin, MU Archangel, and MU Monarch. A mobile game, called MU New Dawn, is currently being published by PlayPark.

The following services is/were marketed to the Philippine region.

• MU Online Philippines' open beta started in 2004, and its commercial launch was in 2005*. There was no account migration when it closed in 2012.
• MU Online Global is listed here, as PlayPark MU's service migrated its player accounts to the global service in 2022.