Monster Arm is the titular main antagonist of the Star vs. the Forces of Evil episode of the same name. He is a monstrous octopus tentacle who was accidentaly conjured up by Star Butterfly so that she can cure Marco's injured right hand.
The Monster Arm at first showed movement of it's own as it had control of itself. It later gained the ability to talk and told Marco to kill Jeremy, one his bullies. Marco realized that the Monster Arm went too far and tried to have Star turn his hand back to normal, but the arm kept on dodging the spells. Marco then told Star to summon the hands on the ground and jumps to were the hands are as they grab the monster arm. Star then casts the hand spell, killing off the monster arm for good, and restoring Marco's normal right hand.
- The Monster Arm is so far the most villainous character in the entire series, even more so than Ludo.
- The Monster Arm might return in some episodes.