In chess, a fortress is an endgame drawing technique in which the side behind in material sets up a zone of protection that the opponent cannot penetrate. This might involve keeping the enemy king out of one's position, or a safe zone the enemy cannot force one out of (e.g. see the opposite-colored bishops example). An elementary fortress is a theoretically drawn position (i.e. a book draw) with reduced material in which a passive defense will maintain the draw.

Fortresses commonly have the following characteristics:

Fortresses pose a problem for computer chess: computers fail to recognize fortress-type positions (unless using a suitable endgame tablebase) and incorrectly claim a winning advantage in them.

Perhaps the most common type of fortress, often seen in endgames with only a few pieces on the board, is where the defending king is able to take refuge in a corner of the board and cannot be forced away or checkmated by the superior side. These two diagrams furnish two classic examples. In both cases, Black simply shuffles their king between a8 and the available square adjacent to a8 (a7, b7, or b8, depending on the position of the white king and pawn). White has no way to dislodge Black's king, and can do no better than a draw by stalemate or some other means.

Note that the bishop and wrong rook pawn ending (i.e. where the pawn is a rook pawn whose promotion square is the color opposite to that of the bishop) in the diagram is a draw even if the pawn is on the seventh rank or further back on the a-file. Heading for a bishop and wrong rook pawn ending is a fairly common drawing resource available to the inferior side.

The knight and rook pawn position in the diagram, however, is a draw only if White's pawn is already on the seventh rank, making this drawing resource available to the defender much less frequently. White wins if the pawn is not yet on the seventh rank and is protected by the knight from behind. With the pawn on the seventh rank, Black has a stalemate defense with their king in the corner.

A fortress is often achieved by a sacrifice, such as of a piece for a pawn. In the game between Gregory Serper and Hikaru Nakamura, in the 2004 U.S. Chess Championship, White would lose after 1.Nd1 Kc4 or 1.Nh1 Be5 or 1.Ng4 Bg7. Instead he played

Heading for h1. After another 10 moves the position in the second diagram was reached. Black has no way of forcing White's king away from the corner, so he played