The Sylverant server is a reverse engineered server for Sega's Phantasy Star Online for the Dreamcast, Gamecube, and PC. The server went live on October 31, 2009.

It originally began as a port of Sodaboy's Phantasy Star Online Blue Burst server "Tethaella" to UNIX-like operating systems, but was re-written for Dreamcast PSO versions to be released in 2009 in celebration of the 10th anniversary release of the Dreamcast in the United States. It uses code from Kohle's PSO server for PC and Gamecube versions, including the suite of quests that are used on the Sylverant server.

Both versions of Phantasy Star Online for the Dreamcast are supported, as well as both versions for Gamecube, and both versions for PC. Xbox versions are not supported, as they are connected to XBOX Live. Note that Gamecube Episode 3 is supported, however not all card functions work. Players from other console versions can talk to each other in the lobbies, but they can not play the game with each other.