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. It opens Spiny Rainbow Romp.
- Spin-Dig Galaxy: Next to the right-most pillar on the Drill Zone Planet. It opens 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. It opens Purple Coin Flip 'n' Sprint.
- Rightside Down Galaxy: Between two Thwomps in the upside-down section, just after the Chain Chomps.
- Bowser Jr.'s Fiery Flotilla: Above the Thwomp near the beginning of the stage. This unlocks Fiery Flotilla Speed Run.