- For other appearences or other robots by this name, see Mecha Sonic.
- For Knuckles' Sky Sanctuary boss fight, see Super Mecha Sonic.
Sonic 3 & Knuckles features a sleek, blue version of Mecha Sonic as the boss of Sky Sanctuary Zone. He is both a boss AND a sub-boss, because in Sonic & Tails' playthrough, the robot is not content to wait at the end of the level for the heroes to come to him. Mecha Sonic harries the animals throughout the zone, flying his master's Egg-O-Matic into battle twice before the real boss fight even occurs! First, Mecha attacks with the ball-and-chain weapon from Green Hill Zone, and later he returns with the simulacrum-producing orbs from Metropolis Zone.
The GHZ Boss sequence plays exactly the same as the original game (albeit with different music, tiles, background and sprites) but the MZ boss sequence is different in that the Egg-O-Matic doesn't fly high enough while there are bubbles around such as to avoid getting hit by the player's jumps, so can be defeated much quicker than the original incarnation.
Only when Sonic climbs to the top of the level does Mecha Sonic deign to fight hand-to-hand (hand-to-servoactuator?). The battle is similar to the one fought in Sonic 2's Death Egg Zone, although Mecha Sonic substitutes the octadirectional quill attack for new backwards-dash and bounce-jump moves. A larger fight arena (and the fact that Sonic is holding rings this time) tends to make this boss a lot easier than its predecessor.
Mecha Sonic is not destroyed by Sonic or Tails here, but rather left downed and sparking as the heroes continue on to the Death Egg. Knuckles finishes the job in his fight against Super Mecha Sonic.