Diagonal Sudoku (Sudoku X) — Play Free Online
Diagonal Sudoku adds one rule to the classic game and changes how the whole board behaves. The two long diagonals, shaded on the grid above, must each hold the digits 1 to 9 exactly once — on top of every row, column and 3×3 box doing the same.
Why one extra rule changes so much
The two diagonals cover seventeen squares, and they cut across all nine boxes. That makes them the only units on the board that are not a neat block or line, and it gives every square on a diagonal three units to satisfy instead of two — four for the centre square, which sits on both.
The practical effect is that the middle of the board, usually the hardest region to open, becomes the easiest. The centre square belongs to a row, a column, a box and both diagonals at once, so it is very often the first digit you can place.
How to use the diagonals
Treat each diagonal as a ninth row. Every technique that works on a row works on it unchanged: scan it for a digit that has only one square left, look for a pair of squares that between them hold only two candidates, and eliminate accordingly.
The one habit worth building is checking the diagonals last on every pass. Most solvers read rows, then columns, then boxes, and simply forget the two extra units — which is why a diagonal grid so often stalls in a position that is not actually stuck. If you cannot find a move, the next move is almost always on a diagonal.
The opposite trick matters too. Because a diagonal is a full unit, the eight squares off it in any row are constrained by what the diagonal already holds. A digit placed on the diagonal removes itself from its row, its column, its box and the whole diagonal at once, which is four eliminations from one placement.
Fewer clues than a classic grid, on purpose
The extra constraint carries real information, so a diagonal puzzle can start with far less and still have exactly one answer. These boards run from 36 clues at the easiest level down to 23 at the hardest, against 42 to 24 on the classic puzzles. Handing a diagonal grid the classic clue counts would make all four levels feel the same, and that sameness would be the puzzle's fault rather than yours.
Every board is checked for a unique solution before it is served, under the diagonal rules rather than the classic ones — a grid that is unique as plain Sudoku can have several answers once the diagonals are added, and the reverse is far more common.
Where this sits next to the other puzzles
All nine solving techniques transfer directly: a hidden single on a diagonal is the same deduction as a hidden single in a row. What does not transfer is the hint button, which is deliberately absent here — the explanation engine reasons about rows, columns and boxes only, so on this board it would describe a game you are not playing.
If you want a variant that changes the rules more sharply, try Killer Sudoku, which removes the given digits altogether and replaces them with cage totals. For a smaller, quicker grid there is 6×6 Sudoku.
Scores on this page are kept off the leaderboard: it is a different puzzle with a different clue range, so its times are not comparable with the classic boards'.
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