World S, also known as World S: Here We Go!, is the seventh and final world in Super Mario Galaxy 2. It is a bonus world. This galaxy is located high in the sky above an unknown colorful planet with colorful star-shaped islands on it. There are several rainbows in the background. The world's path is shaped like an S, which is possibly the reason why it is called World S. It can also be called World S to stand for Special World, similar to how the bonus cups of the Mario Kart series are called the Special Cup. World S is unlocked after defeating Giant Bowser in Bowser's Galaxy Generator, which is located in World 6. This world has seven galaxies, the last being the Grandmaster Galaxy. The Grandmaster Galaxy can only be entered after all 120 Power Stars and 120 Green Power Stars have been collected. The existence of this world is hinted by a Bob-omb Buddy in the Throwback Galaxy, who asks about a "strange, legendary world" in a similar way to how Lubba does so on Starship Mario. This world's music is a slowed-down version of the Gusty Garden Galaxy's music in the prequel.
- Mario Squared Galaxy
- Rolling Coaster Galaxy
- Twisty Trials Galaxy
- Stone Cyclone Galaxy
- Boss Blitz Galaxy
- Flip-Out Galaxy (this galaxy is unlocked by feeding Star Bits to a Hungry Luma on the map)
- Grandmaster Galaxy
- All the galaxies of this world (excluding the Flip-Out Galaxy) are references to the previous 3D Mario games. The Mario Squared Galaxy, Stone Cyclone Galaxy, Boss Blitz Galaxy, and Grandmaster Galaxy are references to Super Mario Galaxy. The Rolling Coaster Galaxy is a reference to Super Mario 64. Last but not least, the Twisty Trials Galaxy is a reference to Super Mario Sunshine.
- This is the only world where every galaxy (excluding the Grandmaster Galaxy) has two main missions, and two Green Power Star missions. The Grandmaster Galaxy has two main missions but lacks Green Star missions since all Green Power Stars in other galaxies must be collected to unlock this galaxy.
- No galaxy in this world has a Teleporter that leads to a Bonus Planet.