Comet Medals are large golden medals that first appear in Super Mario Galaxy 2. Their design is similar to a white octahedron-shaped coin with a golden Prankster Comet embroidered on them. When Mario and Luigi obtain these items in a galaxy, Prankster Comets appear in the galaxy, allowing special stars to be earned. There is one comet medal in each galaxy, and these medals can only be found in the first mission. A similar collectible called a Star Medal appears in Super Mario 3D Land.
- Sky Station Galaxy: High above the cylinder planet with the grey moving platforms. It unlocks Pee Wee Piranha's Speed Run.
- Yoshi Star Galaxy: On a small green hill in the area where the player first obtains Yoshi. Unlocks Spiny Rainbow Romp.
- Spin-Dig Galaxy: Next to the right-most pillar on the Drill Zone Planet. Unlocks Digga-Leg's Daredevil Run.
- Fluffy Bluff Galaxy: Above a column of coins on the Pond Planet. Cloud Mario is needed, or a triple jump or back-flip and spin at the apex of the jump. Fluffy Bluff has no Prankster Comet of its own until the Green Stars are unlocked.
- Flip-Swap Galaxy: On a blue platform near the last Chain Chomp. Unlocks Purple Coin Flip 'n' Sprint.
- Rightside Down Galaxy: Between two Thwomps in the upside-down section, just after the Chain Chomps. This galaxy has no prankster comet.
- Bowser Jr.'s Fiery Flotilla: Above the Thwomp near the beginning of the stage. Unlocks Fiery Flotilla Speed Run.
- Puzzle Plank Galaxy: Off to the side of a wooden platform, just before two Grinders cut out a section of the wall. Unlocks Purple Coin Shadow Vault.
- Hightail Falls Galaxy: Near the end of the level right after the Hungry Luma. Unlocks Hightail Falls Speed Run.
- Boulder Bowl Galaxy: Inside the giant, pink crystal to the left of the starting planet which can be broken after being hit from the Rock Mushroom 2 times. Unlocks Rolling Crabber Romp.
- In the beta stages of the game, the Star imprinted on the Comet Medal lacked eyes. Additionally, the star had a slightly different shape.
- When a collected Comet Medal is recollected, it will still be added to the total Comet Medal count as seen in the Pause Menu. This Comet Medal will be subtracted from the total count once the mission has been completed or exited.
- This makes it possible to effectively possess a total of 50 Comet Medals (temporarily).