Tree of Savior (also known as TOS) is a free massively multiplayer online role playing game developed by IMC Games. The game was developed by Kim Hakkyu, creator of Ragnarok Online while the game's background music was done by various groups and artists like SoundTeMP, the same team known for their soundtracks in Ragnarok Online and Granado Espada. TOS is based on traditional Lithuanian culture and mythology.

Tree of Savior was first revealed at Hangame Ex in 2011 under the code name "Project R1", leading to the attainment of the title "spiritual successor of Ragnarok Online". The game was posted to Steam Greenlight on May 12, 2015 and approved in less than 10 hours. The International version was released through Steam as a Founder's Early Access game on March 28, 2016, with the game releasing as a free-to-play title on April 28, 2016.