Type any of the following codes on the keyboard while in any of the game's front: end menus. A notification popup should appear on screen notifying you of a successful cheat input.

There are two types of codes: CHEATS and OUTFITS. CHEATS can be turned on and off, and must be reactivated whenever the player returns to the main menu after a game: over (though will stay activated if selecting Play Again from Game Over). OUTFITS immediately change the appearance of the Nyan Cat itself.


Cheats:

FunkyFace: Modifies the bottom: screen text notifications in game
OmNomNomable: Changes "Nyans" to "Noms" in gameplay
VertigoKitty: Flips the galaxy upside: down
ColdWintersDay: Makes Nyan Cat move slowly from lane to lane
DownInFront: Moves Nyan Cat far up the screen
SnookiesCookies: Changes the word "cookies" to "Snookies" in the text notifications
ReverseEngineering: Reverses the galaxy horizontally, and inverts the rainbow trail
TastingTheRainbow: Reverses Nyan Cat
NyanCatMarch: Replaces bottom: screen text notifications with a parade of Nyan Cats
UseYourWhiskers: Disables the HUD
Leprechaunitis: Disables Nyan Cat's rainbow
CatBreadForTheWin: Replaces all yummies with Cat Bread!
InvisibilityCloak: Makes Nyan Cat completely invisible


Outfits:

BlackAlbum: Makes Nyan Cat completely black
WhiteAlbum: Makes Nyan Cat completely white
DreamsComeTrue: Makes Nyan Cat flash rainbow colors
SummerNyan: Turns Nyan Cat into a watermelon
ChocoKittyTart: Makes a Chocolate Pop: Tart Themed Nyan Cat
YouLookLikeAPopTart: Makes Nyan Cat white, pink and purple
NyanOLantern: Makes Nyan Cat Hallowe'en: Themed
NyanFisher: Makes Nyan Cat stealth themed (a la Sam Fisher)
GGGGhostNyan: Makes Nyan Cat transparent
NyanTheDarkLord: Makes Nyan Cat black and glow red. Red glow is retained after reloading the game, black is not.
NyanStamp: Embosses Nyan Cat
ForgotMyGlasses: Makes Nyan Cat blurry and nearly invisible
DivideByZero: Makes Nyan Cat invert the color of whatever is behind it