Demons are supernatural and often malevolent beings prevalent in religion, occultism, literature-fiction, mythology and folklore.
The appearance of demons in the various TMNT universes has become a semi-recurring occurrence.
- Savanti Romero was once human, but was transformed into a demon by Lord Simultaneous as punishment for his crimes.
- "Halls of Lost Legends" - Loki summons the demon Grendel to fight the Turtles.
- "Raphael: Bad Moon Rising" features a demon named Lord Incubor, an inhabitant of the world of Nocturna.
- "The Burning Man" - The Foot Clan summons The Burning Man.
- Null, a recurring adversary of the Turtles, is a demonic businessman who claims to be the Devil himself.
- Noi Tai Dar is a demon released to Earth by Chien Khan.
- Season 5 of the 2012 series introduces Kavaxas, the lord of the Demodragons, a race of demonic dragons.