"Spookwave" is the 39th track in the Undertale Soundtrack. It has the same melody as "Spooktune," but with a phaser/flanger effect. It is in the second CD found in Napstablook's house. The song continues playing after exiting their house. This can trigger an event where the protagonist encounters Aaron and Woshua, who are too scared to fight due to the spooky music. It consists of a single sample.
- "Spookwave" is a reference to the music genre vaporwave.
- "Spookwave" also sounds very similar to the theme from Pinky and the Brain when sped up slightly. This could be a reference to how Vaporwave commonly consists of slowed down samples of other songs.
- Like Spooktune, the game data file names this song "
mus_spoopy_wave.ogg," a reference to the infamous "spoopy" meme.