|This article is about the character. You may be looking for the episode.|
- “You'll never get rid of me! I'm part of you now! You can treat the symptoms, but you'll never cure the virus!”
- ―Monster Arm's last words[src]
It has a mind of its own and desires to eat the bowels of humans. Once finding out about its intentions and its bad influence on him, Marco gets Star to fix it and return his arm to normal. Monster Arm, before it's completely gone, threatens that it isn't permanently removed and it will remain a part of Marco.
Before it revealed itself as sentient, the Monster Arm would spill Marco's food, slap him in the face, and mess with his shower water, apparently for no reason.
Monster Arm is a large tentacle creature with a pink underside and light pink suction cups. Most of the arm is deep blue and fades to purple at the tip of the tentacle. Its teeth are dull yellow and sharp, and it is covered in pink slime.
It is strong enough to allow Marco to do 675 one-arm (one-tentacle) handstand push-ups with it.
- Theme Song (season 1)
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- Monster Arm appears in the crowd surrounding Star and Marco in the season 1 opening theme song.
- Monster Arm appears in "Storm the Castle" in a brief flashback Marco has when Star tells him to put his arm inside a strange creature.