Eufonius is a Japanese progressive pop rock music group which debuted on October 11, 2003 with the release of their first album Eufonius which was also an independent release. The band has produced songs for various anime and video games, such as Kashimashi: Girl Meets Girl, Himawari!, Noein, True Tears, Yosuga no Sora, Kokoro Connect, Shinkyoku Sōkai Polyphonica and visual novels like Key's Clannad and Favorite's Irotoridori no Sekai. The band is composed of two primary members, Riya—who provided vocals and the majority of the lyrics—and Hajime Kikuchi—who plays the keyboard and handles composition, arrangement, and sometimes the lyrics. The third member is Masashi Ōkubo, who is a supporting member of the group.

They have released 15 albums as of August 2014, and have provided their talents in compilation albums and original soundtracks as well. Eufonius has hosted an Internet radio show called Frequency-e since May 2005 with broadcasts being released in irregular intervals; as of December 2011, there have been 12 broadcasts. On September 8, 2012, it was announced that Hajime Kikuichi would leave the band "for now", after making some degrading comments at singer Halko Momoi. Kikuchi returned in 2013 with the composition of the song "Lovely Smile", used as a theme song of Hooksoft's visual novel Lovely Quest: Unlimited.