Card marking is the process of altering playing cards in a method only apparent to marker or conspirator, such as by bending or adding visible marks to a card. This allows different methods for card sharps to cheat or for magicians to perform magic tricks. To be effective, the distinguishing mark or marks must be visible on the obverse sides of the cards, which are normally uniform.

Card marking is often used to cheat when gambling or for card tricks. Many casinos, particularly those in Las Vegas alter the decks of cards they sell to tourists – either by punching holes through the middle of cards or trimming their edges – to prevent cheaters from returning to the game tables after buying the cards and then slipping the favorable cards into their hands when playing.

Marked cards can be used regardless of who shuffles and deals the cards. Some more sophisticated marked-cards scams involve additional manipulative skills to steer the cards into the correct positions once the desired cards have been identified.