Castling is a move in chess. It consists of moving the king two squares toward a rook on the same rank and then moving the rook to the square that the king passed over. Castling is permitted only if neither the king nor the rook has previously moved; the squares between the king and the rook are vacant; and the king does not leave, cross over, or finish on a square attacked by an enemy piece. Castling is the only move in chess in which two pieces are moved at once.

Castling with the king's rook is called kingside castling, and castling with the queen's rook is called queenside castling. In both algebraic and descriptive notations, castling kingside is written as 0-0 and castling queenside as 0-0-0.

Castling originates from the king's leap, a two-square king move added to European chess between the 14th and 15th centuries, and took on its present form in the 17th century. Local variations in castling rules were common, however, persisting in Italy until the late 19th century. Castling does not exist in Asian games of the chess family, such as shogi, xiangqi, and janggi, but it commonly appears in variants of Western chess.

During castling, the king is shifted two squares toward a rook of the same color on the same rank, and the rook is transferred to the square crossed by the king. There are two forms of castling:

Castling is permitted provided all of the following conditions are met:

Conditions 3 and 4 can be summarized by the mnemonic: A player may not castle out of, through, or into check.

Castling rules often cause confusion, even occasionally among high-level players. Alexander Beliavsky and Viktor Korchnoi both had to consult the arbiter during tournaments on whether castling was legal when the rook was on or passed over an attacked square, Yuri Averbakh once mistakenly thought that Black queenside castling was illegal when b8 was attacked, and Nigel Short once attempted to castle queenside as Black when d8 was under attack (this was not allowed). Illegal castling has also occasionally occurred in serious games between top players (including Gata Kamsky, Viktor Korchnoi, and Richard Réti) when they forgot that the king or rook had previously moved and returned to its home square, and has not always been noticed by the opponent. Yasser Seirawan once accidentally castled queenside as White with his queen's rook on b1 (which was not allowed), and Alexander Alekhine once "castled his queen" (moving his queen from d1 to b1 and his rook from a1 to c1, which was also not allowed). To clarify:

Under FIDE rules and USCF rules, and enforced in most tournaments, castling is considered a king move, so the king must be touched first; if the rook is touched first, a rook move must be played instead. As usual, the player may choose another legal destination square for the king until releasing it. When the two-square king move is completed, however, the player is committed to castling if it is legal, and the rook must be moved accordingly. The entire move must be completed with one hand. A player who attempts to castle illegally must return the king and rook to their original squares and then make a legal king move if possible (which may include castling on the other side). If there is no legal king move, the touch-move rule does not apply to the rook.