X-Sudoku

X-Sudoku. Both main diagonals must also contain digits 1 to 9 with no repeats.

How to play

Fill the 9×9 grid so every row, every column, and every 3×3 box contains the digits 1 to 9 with no repeats — the classic Sudoku rule. X-Sudoku adds one twist: Both main diagonals must also contain digits 1 to 9 with no repeats.

X-Sudoku, also called Diagonal Sudoku, keeps the ordinary 9×9 grid but promotes both main diagonals into extra houses: the nine cells running corner to corner in each direction must also hold 1 to 9 with no repeats. The two diagonals cross at the very centre and trace out the "X" the variant is named for, layering two more nine-cell constraints on top of the usual rows, columns and boxes. Because those diagonals bite in places the plain grid never touches, an X-sudoku puzzle can be set from fewer starting clues than a classic one of the same difficulty.

How to solve X-Sudoku

Treat each of the two long diagonals as an extra nine-cell house and scan it for naked and hidden singles exactly as you would a row, column or box — a digit that can legally sit in only one of a diagonal's nine cells belongs there. Begin at the centre, R5C5, which lies on both diagonals as well as on its own row, column and box: with five separate 1-to-9 houses bearing on it, it is the most constrained square on the board and often resolves before anything else. Every digit you drop onto a diagonal is erased from the other eight cells of that diagonal, so one placement ripples across the whole line and frequently uncovers a hidden single the plain grid would never force. Watch the cells where a diagonal cuts through a corner box, because a digit locked to a box's diagonal cells clears from the rest of that box, and a digit pinned to those same cells clears from the rest of the diagonal. Resolve the two diagonals early and their leftover eliminations tend to unlock the surrounding rows and columns in turn.

Every X-Sudoku puzzle is free to play in your browser — no ads, no sign-up, and it works offline.

Frequently asked questions

What is X-Sudoku?

X-Sudoku, sometimes called Diagonal Sudoku, is a 9×9 variant in which the two main diagonals — the lines running from one corner of the grid to the opposite corner — must each contain the digits 1 to 9 with no repeats, in addition to the usual row, column and box rules. The two diagonals cross to form an 'X' across the grid, which is where the name comes from. They give you two more nine-cell houses to reason with, which is why a valid X-Sudoku can start from fewer clues than a classic puzzle of the same rating.

What are the rules of X-Sudoku?

Every row, column and 3×3 box must contain 1 to 9 exactly once, just as in classic Sudoku. X-Sudoku adds a single extra rule: each of the two main diagonals must also contain 1 to 9 with no repeats — the diagonal from the top-left corner down to the bottom-right, and the one from the top-right corner down to the bottom-left. Cells that lie off both diagonals answer only to their row, column and box, while the diagonal cells carry that extra fourth constraint.

Why start at the centre cell in X-Sudoku?

The centre cell, R5C5, is the only square that sits on both diagonals at once, as well as on its own row, column and box. That subjects it to five separate 1-to-9 houses instead of the usual three, so more candidates are eliminated there than anywhere else on the grid. It is frequently the first cell to collapse to a naked or hidden single, and fixing it feeds eliminations into both diagonals at the same time, which is why experienced solvers look at it first.