- Mathematician
- Mausoleum of the Emperor
- Max Warrior
- Maximum Six
- Mecha Ojama King
- Mecha Ojama King Transformation
- Mechanicalchaser
- Medium Piece Golem
- Medusa Worm
- Mefist the Infernal General
- Megamorph
- Megazowler
- Meklord Army of Granel
- Meklord Army of Skiel
- Meklord Army of Wisel
- Meklord Astro Dragon Asterisk
- Meklord Astro Mekanikle
- Meklord Emperor Skiel
- Melchid the Four-Face Beast
- Melomelody the Brass Djinn
- Mercury Hourglass
- Mermaid Knight
- Mesmeric Control
- Messenger of Peace