Metal Knuckles is a robotic version of Knuckles who appeared in Sonic R as an unlockable character. It was created by Dr. Robotnik, and its abilities are similar to Knuckles'.

To unlock him in Sonic R, the player needs to collect all 5 Sonic tokens in Reactive Factory, finish in at least 3rd, then beat him in a one-on-one race.

Sonic Channel featured a Japanese profile of Metal Knuckles. Translated, it states,

"As with Sonic, Eggman prepared this secret weapon for the sake of overthrowing Knuckles, considering his existence to be a hindrance. Having the same power plant as Metal Sonic, speed and acceleration power are high, but like Knuckles, this robot can glide from high places."

Appearance requirements: Collect 5 tokens in Reactive Factory in Grand Prix mode and finish better than fourth place to begin a one-on-one match with Metal Knuckles. Win this race and Metal Knuckles will become a possible choice on the character select screen.

Metal Knuckles is just like Knuckles  The Echidna but with green eyes.