Perfect Chaos is the final boss of Sonic Adventure, when the player has completed all six stories for each playable character. It takes place in the flooded Station Square with a cloudy sky and has two phases. It is also the only moment in the game to play as Super Sonic, who is more fast moving than his regular form. The path between the player and Perfect Chaos is a long route, where the player is only able to move during the boss battle. Some of the roadways from Speed Highway are also seen there as well, where the player is able to move. The route differs slightly depending on if the player succeeds or fails to hit Perfect Chaos' weak spot. In either case, the player moves to another section to do the same pattern.
Right at the start, the player has fifty rings on hand. Super Sonic is able to hover normally in the water. The player's goal is to gain enough speed to move towards Perfect Chaos. The playable character will automatically Spin Attack inside the beast. If the player manages to gain enough speed, he/she will spin to Perfect Chaos' weak spot, its brain, and the player moves to the next route to do the same thing. However, if the player hasn't gained enough speed, the player will fly away and has to try again in another place.
From the beginning, the player has to watch their ring count, which typically, in Super Sonic's case, will decrease one ring per second. The player can collect rings on strings, that are set on roadways with Dash Panels, which can gain more speed for the player. Super Sonic is also able to gain a small amount of speed by jumping and moving forward. When the player has gain enough speed, Super Sonic will perform a Bolt of Lightning. If the ring count manages to go zero before completing the boss battle, the player will lose a life and try again at the beginning of the battle.
Perfect Chaos has three major attacks to use against the player. The first one is shooting large, energy projectiles from its spires on its back towards the player. It can also summon large typhoons near him and its final attack is a large energy beam that it shoots from its mouth, which it used to destroy Egg Carrier 2. If the player gets hit by one of these attacks, he will get fly backwards and loses all the maximum speed that has been gained. During the first phase, the player has to hit Perfect Chaos' brain three times while the game's main theme Open Your Heart can be heard in the background. In the second phase, a more darker, orchestral theme called "Perfect CHAOS Revival!" can be heard in the background, while the battle becomes quite harder and the player has to hit the same target another three times. After this, the player has completed the game.
The Perfect Chaos battle in Sonic Adventure only takes about six hits to defeat, despite it being a final boss.
There is unused attack by Perfect Chaos seen in the game files where Perfect Chaos launches a small arm from the floods to attack Super Sonic, though the attack is not fully finished.