What is Diagonal Sudoku?
Diagonal Sudoku, also known as X-Sudoku or Sudoku X, adds one simple rule to the classic game: the two long diagonals, shaded on the board, must each contain the digits 1 to 9 exactly once.
The extra constraint means puzzles can start with fewer numbers while staying fair. It is the easiest variant to pick up if you already enjoy classic Sudoku.
Rules
- Each row, column and 3×3 box contains the digits 1 to 9 exactly once.
- Each of the two main diagonals (shaded) also contains 1 to 9 exactly once.
Diagonal Sudoku strategy
- Treat each diagonal as a tenth and eleventh unit: scan them for hidden singles just like rows and boxes.
- The centre cell lies on both diagonals, the middle row and the middle column — it is often the key to the puzzle.
- Diagonal cells see more of the board than ordinary cells, so their candidate lists shrink fast. Start your notes there.
New to Sudoku logic? Start with our guide to playing Sudoku and the tips and strategy page — every classic technique works in this variant too.
Frequently asked questions
What is X-Sudoku?
Another name for Diagonal Sudoku: a classic Sudoku in which both long diagonals must also contain 1 to 9 once.
Is Diagonal Sudoku harder than classic Sudoku?
The extra rule gives you more information, so the puzzles start with fewer numbers. Our levels are graded by the techniques required, so Easy stays easy.
Do hints work in Diagonal Sudoku?
Yes. The hint system understands the diagonals and will point to them when they are the key to the next move.