Cheating in bridge refers to a deliberate violation of the rules of the game of bridge or other unethical behaviour that is intended to give an unfair advantage to a player or team. Cheating can occur in many forms and can take place before, during, or after a board or game.

Commonly cited instances of cheating include: conveying information to a partner by means of a pre-arranged illegal signal, viewing the opponents' cards in a board prior to their arrival at the table, altering the records as to the results of a board; in certain games, it may include illicit shuffling to deal favourable cards to oneself or one's partner or marking cards so as to make their denomination and/or rank apparent to the perpetrator.

Unlike games such as chess which provide perfect information to the competitors, bridge is a game of imperfect information, and the exact contents of partner's and opponents' hands remain unknown until later in the play. Players are only entitled to act upon information conveyed by calls made and cards played, along with the visible contents of their own hand (and dummy's hand when exposed). Unauthorized information (UI) (Law 16) is any information that a player obtains by means of:

Law 16 of the Laws of Duplicate Bridge states that Players are authorized to base their calls and plays on information from legal calls and plays and from mannerisms of opponents. To base a call or play on other extraneous information may be an infraction of law.

The highest bridge ethics, expected from the top players, require that the players take extra steps neither to convey nor to act on any unauthorized information. However, as the stakes in terms of prize money, sponsor money and ranking are often high, throughout the history of the game there have been a number of players who have used illegal methods in order to gain an advantage over their opponents. A notable innovation in the 1960s and 70s was a move away from classic oral bidding at serious bridge events toward use of bidding boxes that contain cards. This allows players to signal their bids in a largely mute and orderly fashion with little opportunity for small talk, barring the use of an alert card wherein the players are in uncharted territory or otherwise using a custom bidding convention and may be required to explain if their opponents ask. This reduces the scope for unscrupulous players to smuggle in signals. An overt example in a casual game with oral bidding might go something like:

Bidder: "Hmm, what to bid."

Partner: "My poor kitty cat got spayed last Tuesday."

Bidder: "My condolences to her. I bid 2 ♠."