For each section of the game, Fox composed the music prior to programming, as it helped 'decide how the scene should go'. 'Megalovania' (stylized as 'MEGALOVANIA'), the song used during the boss battle against Sans, had previously been used within Homestuck and in one of Fox's EarthBound ROM hacks. According to Fox, over 90% of the songs were composed specifically for the game.

Fox also stated that he tries to be inspired by all music he listens to, particularly those in video games. The soundtrack was inspired by music from Super NES role-playing games -such as EarthBound and Live A Live - bullet hell series Touhou Project, the chiptune band Anamanaguchi, as well as the webcomic Homestuck, for which Fox provided some of the music. A self-taught musician, he composed most of the tracks with little iteration the game's main theme, 'Undertale', was the only song to undergo multiple iterations in development. The game's soundtrack was entirely composed by Fox with FL Studio.

Undertale Soundtrack (stylized as UNDERTALE Soundtrack) is a soundtrack album by Toby Fox, released in 2015 for the video game Undertale.

Undertale soundtrack mp3