In short: A move the rules don't allow, such as a move that leaves your own king in check. In a tournament your opponent gets extra time the first time, and the second time you lose.
An illegal move is a move the rules of chess don't allow. It can be a move that leaves your own king in check, for example with a piece pinned to the king, or castling through an attacked square. In the middle of an exciting game it happens to everyone sooner or later.
What happens in a tournament
The move counts as made once you have pressed the clock. If it is discovered afterward, the position before the move is put back and you make a new move. The touch-move rule applies: you must move the piece you touched, if you can.
- The first time, your opponent gets two minutes of extra time.
- The second time, you lose the game.
- The exception: if your opponent can't checkmate you by any possible series of legal moves, the game is drawn instead of lost.
The same penalty applies if you make a move with two hands, such as castling or a capture, and if you press the clock without having moved. If a pawn has reached the last rank and you press the clock without replacing it, that is illegal too, and the pawn becomes a queen.
In blitz and rapid
The penalty is one minute instead of two, except in blitz where each game has its own arbiter who records the moves. There the full competition rules apply, and the penalty is two minutes. If no arbiter is watching your game, your opponent must point out the illegal move before making their own next move. If they don't, the move stands and the game goes on.
At the kitchen table you make your own rules. A friendly "you can't do that, your king is in check" usually goes a long way. The rules above are FIDE's, in the version in force since January 1, 2023.
Want to go over what is allowed? Start with the rules of chess and the page on check and checkmate.
Frequently asked questions
When does an illegal move count?
Once you have pressed the clock. Before that you can still correct it, but you must still move the piece you touched, if you can.
What are the rules in blitz and rapid?
The penalty is one minute instead of two, except in blitz where each game has its own arbiter who records the moves: there the full competition rules apply. And if no arbiter is watching your game, your opponent must point out the illegal move before making their own next move. Otherwise the move stands.
Is it illegal to use two hands?
Yes. If you make a move with two hands, castling for example, and press the clock, it is penalized as an illegal move. The same goes for pressing the clock without having moved.
Updated November 14, 2026