Knuckles the Echidna is a boss in Sonic & Knuckles, as well as its locked-on form, who is encountered in Hidden Palace Zone. In this battle, Knuckles challenges Sonic the Hedgehog (and/or Miles "Tails" Prower) to a fight.
Ever since Robotnik crashed the Death Egg into Angel Island, Knuckles has been tricked into thinking Sonic and Tails were after the Master Emerald; and now, in Hidden Palace Zone, the echidna makes his final defense of the Emerald's shrine.
Knuckles' attacks include the Spin Dash, gliding, and falling on the player. If the player gets too close to Knuckles, he may use a two-punch combo to attack the player. If the player attempts to use the Spin Dash on Knuckles, he will deflect the attack, causing the player to bounce back in the opposite direction. To defeat Knuckles, the player must Spin Jump on Knuckles' head while he is standing, or from beneath him if he is gliding. Knuckles will also suffer damage if he falls on the player while they are using the Spin Dash.
After eight hits, Knuckles is temporarily knocked out. The ominous sound of a disturbance from the Emerald's shrine, however, wakes him up; Sonic and Tails follow Knuckles to watch Robotnik steal the Master Emerald in his Egg Mobile. Knuckles then gets shocked by Eggman.
- This was the first Sonic boss that is not an Eggman robot (with the possible exception of Sandopolis' Guardian).
- In Angel Island Zone in Sonic Advance, the player fights Mecha Knuckles in a similar manner to the Knuckles boss fight.
- During the battle at Hidden Palace, a mural in the background can be seen depicting a spiky, blue creature surrounded by a fiery aura fighting a large, mustached, humanoid figure for possession of the Master Emerald. This alludes to the final battle of Sonic & Knuckles in the Doomsday Zone.
- This boss is very similar to the Knuckles fight from Sonic Pocket Adventure.