Phantasy Star Online (Japanese: ファンタシースターオンライン HepburnFantashī sutā Onrain ?) is a series of massively online multiplayer action RPG video games released by Sega. It is a subset of Sega's Phantasy Star series of games that began in 1987.

The PSO series began with the release of Phantasy Star Online on the Dreamcast in 2000, and was followed in 2001 by Phantasy Star Online ver. 2, also initially released for the Dreamcast and later ported to Microsoft Windows. In 2002, Phantasy Star Online Episode I & II was released for the Nintendo GameCube and Microsoft Xbox. Subsequently, Phantasy Star Online Episode III: C.A.R.D. Revolution was released for the GameCube in 2003, using gameplay mechanics that were different from the preceding titles. Phantasy Star Online: Blue Burst, a port of the Xbox version of Episode I & II with the addition of a new Episode IV, was released for Microsoft Windows in 2004 through digital distributionPhantasy Star Online 2 was released in Japan for Windows in July 2012, and released for the PlayStation Vita in late 2012 and is due to be released for theiOS and Android in 2013.

With the exception of Episode III, the PSO games are hack and slash role-playing games in which the player slays monsters, levels up, buys new equipment, etc. The PSO games differ from earlier Phantasy Star titles by offering a real-time – rather than turn-based – approach to combat, seamlessly integrating this with exploration and plot developments.