Ultimate Tic Tac Toe is a variation of the classic game of Tic Tac Toe, but with a twist. The game is played on a 3x3 grid, with each square in the grid containing another 3x3 grid. The objective of the game is to win three smaller Tic Tac Toe games in a row.

Here are the rules of Ultimate Tic Tac Toe:

1. Players take turns placing their mark (X or O) in one of the smaller 3x3 grids. Each small grid corresponds to the larger grid. If one player plays in the top right corner in any smaller grid the next player has to play in the top right corner in the larger grid.
2. The first player to win three smaller Tic Tac Toe games in a row (horizontally, vertically, or diagonally) wins the overall game.
3. If a player wins a smaller Tic Tac Toe game, they can choose any square in the larger 3x3 grid to place their mark next.
4. If a player's move in a smaller grid sends their opponent to a grid that has already been won, the opponent can choose any open square on the board.
5. If a player's move in a smaller grid sends their opponent to a grid that has already been tied, the opponent can choose any open square on the board.

Overall, the rules of Ultimate Tic Tac Toe are similar to regular Tic Tac Toe, but with the added strategy of aiming to win multiple games in a row.