Diagonal Sudoku (Sudoku X) - Play Free Online

Diagonal Sudoku (also called Sudoku X) takes the classic 9×9 puzzle and adds two extra constraints: both main diagonals must contain digits 1 through 9 exactly once. This raises the total constraint groups from 27 (9 rows + 9 columns + 9 boxes) to 29. The top-left-to-bottom-right diagonal and the top-right-to-bottom-left diagonal each act as an additional region alongside your usual rows, columns, and boxes.

Interestingly, the extra rule makes things both harder and easier. Harder because diagonal cells have an extra uniqueness requirement to satisfy. Easier because those same cells now have more peers, giving you more information per placement. The net effect: a tight, satisfying logic challenge.

Standard Sudoku rules still apply: 9×9 grid, every row, column, and 3×3 box gets digits 1–9 once.

On top of that, the main diagonal (top-left to bottom-right: r1c1, r2c2, … r9c9) needs each digit 1–9 exactly once.

Same goes for the anti-diagonal (top-right to bottom-left: r1c9, r2c8, … r9c1). Worth noting: the center cell at r5c5 sits on both diagonals, making it the single most constrained cell on the board.

The center cell (r5c5) is your starting point. It belongs to a row, column, box, and both diagonals - five constraint groups. Narrow it down early and you'll unlock progress across the whole board.

After that, focus on diagonal cells. Each one has peers in its row, column, box, and the full diagonal - up to 32 unique peers versus the usual 20. More peers means more eliminations with every placement.

Be aggressive with pencil notes on diagonal cells. Placing a digit on any diagonal cell lets you eliminate it from every other cell on that diagonal. Layer this on top of standard techniques and you'll cut through the puzzle efficiently.

Start your scans with diagonal cells. The extra constraints mean they're often the first to resolve, giving you a foothold in the puzzle.

The center cell is your best friend. It belongs to five constraint groups (row, column, box, both diagonals). Crack it early and you'll slash candidates across the board.

Keep an eye out for diagonal-only eliminations. Sometimes a digit gets ruled out on a diagonal cell purely from the diagonal constraint, even when the row, column, and box would still allow it. Easy to miss, but incredibly useful.

Regular Sudoku has 27 constraint groups: 9 rows, 9 columns, 9 boxes. Diagonal Sudoku adds 2 more (both diagonals) for 29 total. That means 17 of the 81 cells sit on a diagonal and have more peers than in a standard grid - up to 32 peers versus the usual 20.

For a given number of givens, many solvers find Diagonal Sudoku slightly easier because the diagonals supply bonus information. Puzzle designers counter this by providing fewer givens, keeping things balanced. You'll also encounter solving patterns that simply don't exist in regular Sudoku.