Nine men's morris is a strategy board game for two players, dating back to at least the Roman Empire. The game is also known as nine-man morris, mill, mills, the mill game, merels, merrills, merelles, marelles, morelles, and ninepenny marl in English. In North America, the game has also been called cowboy checkers, and its board is sometimes printed on the back of checkerboards. Nine men's morris is a solved game, that is, a game whose optimal strategy has been calculated. It has been shown that with perfect play from both players, the game results in a draw.

The Latin word merellus means 'gamepiece', which may have been corrupted in English to 'morris', while miles is Latin for soldier.

Three main alternative variations of the game are three, six, and twelve men's morris.

The board consists of a grid with twenty-four intersections, or points. Each player has nine pieces, or men, usually coloured black and white. Players try to form 'mills'—three of their own men lined horizontally or vertically—allowing a player to remove an opponent's man from the game. A player wins by reducing the opponent to two men (whereupon they can no longer form mills and thus are unable to win) or by leaving them without a legal move.

The game proceeds in three phases:

The game begins with an empty board. The players determine who plays first and then take turns. During the first phase, a player's turn consists of placing a man from the player's hand onto an empty point. If a player is able to place three pieces on contiguous points in a straight line, vertically or horizontally, they have formed a mill, which allows them to remove one of the opponent's pieces from the board. A piece in an opponent's mill, however, can be removed only if no other pieces are available. After all men have been placed, phase two begins.

Players continue to alternate moves, this time moving a man to an adjacent point each turn. A piece may not "jump" another piece. Players continue to try to form mills and remove the opponent's pieces as in phase one. If all a player's pieces get blocked in (where they are unable to move to an adjacent, empty space) that player loses. A player can "break" a mill by moving a piece out of an existing mill, then moving it back to form the same mill a second time (or any number of times), each time removing one of the opponent's men. The act of removing an opponent's man is sometimes called "pounding" the opponent. When one player has been reduced to three men, phase three begins.

When a player is reduced to three pieces, there is no longer a limitation on that player of moving to only adjacent points: The player's men may "fly" (or "hop", or "jump") from any point to any vacant point.