Diagonal Sudoku

Diagonal Sudoku — published in puzzle magazines as Sudoku X — is a free online Sudoku variant you can play instantly in your browser, no download and no account needed. Every row, column and 3x3 box still needs the digits 1 to 9 exactly once, and two extra units join them: the two main diagonals running corner to corner. Both are shaded on the board so the extra rule is always visible, and selecting a diagonal cell lights up the rest of its diagonal. Twelve levels strip clues from 47 givens down to 23, with undo, three hints and a replayable board on every level.

How to play Diagonal Sudoku

1. Tap any empty cell to select it, then tap a digit from 1 to 9 on the number pad to fill it in.
2. As in classic Sudoku, every row, every column and every 3x3 box must contain each digit from 1 to 9 exactly once.
3. The extra rule: the two shaded diagonals running corner to corner must each contain 1 to 9 exactly once too. The centre cell sits on both, so it is constrained twice.
4. A wrong digit is flagged in red right away, so you can undo or erase it before it throws off the rest of the grid.
5. Fill every cell legally to clear the level and unlock the next one. Later levels start with far fewer given digits.

Tips

• Start on the diagonals, not the rows. A diagonal crosses all nine boxes worth of constraints, so its nine cells are usually the most over-constrained on the board and often crack first.
• The centre cell belongs to a row, a column, a box and both diagonals at once — five units. If any digit is solvable early, it is usually that one.
• Where a diagonal passes through a corner box it takes three of that box's nine cells, which frequently forces the remaining six into a narrow set of options.
• Use the number pad as a checklist: a digit greys out once it has been placed nine times, so you know it is finished everywhere.
• Stuck on a late level? Undo backs out a bad guess without losing the board, and a hint reveals one correct cell — you get three per level.

FAQ