SuperMarioGlitchy3, more commonly abbreviated as SMG3, is a classic major character and the overall main antagonist of the first 10 seasons of the SMG4 series. He initially served as one of two main antagonists of the earlier seasons of the series (alongside Bowser), and the main antagonist of The YouTube Arc in Season 10 until his banishment to the Internet Graveyard in SMG4: War Of The Fat Italians 2020.

He is SMG4's evil doppelganger who was first discovered trying to copy SMG4's videos, and had attempted to kill him ever since.