Diagonal Sudoku (Sudoku X)

Classic Sudoku with a twist: as well as every row, column and 3×3 box, the two main diagonals must each hold the numbers 1 to 9. The diagonals are shaded on the board, so the extra rule is always in view. Fresh, unique puzzles in three levels — free, right in your browser.

What makes it "X"

Diagonal Sudoku, often written Sudoku X, adds a single rule to the puzzle you already know. On top of the usual constraints, the two long diagonals that cross the grid corner to corner must each contain all nine numbers with none repeated. That one addition changes how the board behaves: the centre and the four corners suddenly belong to an extra group, and numbers ripple along the diagonals in ways they never do in plain Sudoku. It is the same clean logic, with a fresh set of patterns to spot.

How to play

Fill the 9×9 grid so that every row, every column, every 3×3 box and both shaded diagonals contain 1 through 9 exactly once. Tap a cell, choose a number, and watch the diagonals as closely as the rows and columns. Because each board is generated with a single guaranteed solution under the diagonal rule, you never have to guess — every step can be reached by logic.

Why the diagonals help you solve

It may sound like extra work, but the diagonal rule usually gives you more to go on, not less. A number that would be ambiguous by row, column and box alone is often pinned down the moment you also check the diagonal it sits on. The nine diagonal cells behave like a tenth and eleventh "line" across the board, so scanning them becomes part of your rhythm. Once it clicks, the diagonals turn into your fastest source of certain moves.

Tips for your first Sudoku X

• Treat each diagonal as its own row: after every placement, ask whether it completes or constrains a diagonal.
• The centre cell is the busiest square on the board — it belongs to a row, a column, a box and both diagonals, so solving it early unlocks a lot.
• Use pencil notes on the diagonal cells first; they tend to resolve quickest.
• Stuck? The built-in hint explains the next logical step, diagonals included.

All the tools you expect

• Pencil notes, hint, undo, redo and erase — all work just as in the classic game, but aware of the diagonals.
• Three difficulty levels, a solve timer, and a fresh, unique board every time.
• Your progress is saved automatically, and the game follows the site's light or dark theme.

Common mistakes to avoid

The one trap that catches most newcomers is forgetting the diagonals once a board gets busy. It is easy to fill a number that is perfectly legal for its row, column and box, yet quietly repeats on a diagonal you were not watching — and because the error can sit several moves back, it is painful to unwind. Build the habit of glancing at both shaded lines after every placement, especially near the centre and the corners, where a single cell can belong to a box and a diagonal at once. The second common slip is treating the two diagonals as interchangeable; they cross only at the middle cell, so a number that resolves one tells you nothing directly about the other except through that shared centre. Keep them separate in your mind and the extra rule becomes a help rather than a hazard.

A brief history of Sudoku X

The diagonal variant is almost as old as the modern puzzle boom. As Sudoku spread through newspapers in the mid-2000s, setters looked for ways to keep seasoned solvers interested, and adding the two main diagonals was one of the earliest and most elegant twists — enough to change the solving experience without adding anything new to learn. It has been a fixture of puzzle pages and apps ever since, usually under the name Sudoku X for the shape the two diagonals trace across the grid.

More ways to play

If you like an added constraint, Killer Sudoku layers arithmetic cages onto the grid, and Flower Sudoku swaps the boxes for jigsaw regions. Prefer a calmer, more visual solve? Try Color Sudoku. For the pure original, the classic board is always a tap away, with a sterner test on Expert Sudoku.