What is X-Sudoku?
X-Sudoku is a 9x9 Sudoku variant. All the standard rules still hold: every row, every column and every 3x3 box must contain the digits 1 to 9 exactly once. The variant adds two constraints along the grid's main diagonals, the lines that run corner to corner and form a giant X.
Each of those diagonals is treated like a ninth-and-tenth region of nine cells: the top-left to bottom-right diagonal must hold 1 to 9 once, and so must the top-right to bottom-left diagonal. The two extra lines are usually shaded so you can see them at a glance.
- Every row contains the digits 1 to 9 once.
- Every column contains the digits 1 to 9 once.
- Every 3x3 box contains the digits 1 to 9 once.
- The main diagonal (top-left to bottom-right) contains 1 to 9 once.
- The anti-diagonal (top-right to bottom-left) contains 1 to 9 once.
How the diagonals change the solve
Treat each diagonal as an extra line to scan, exactly like a row or column. Whenever you place a digit on a diagonal, you can immediately rule it out of the other eight cells on that diagonal — even though they sit in different rows, columns and boxes.
That extra reach is what makes X-Sudoku special. A single placement near a corner can ripple all the way across the grid, and a digit that looks free in standard Sudoku is often pinned down the moment you remember the diagonal it belongs to.
The centre cell and the central box
The middle cell, r5c5, is the only square that sits on both diagonals at once. It is constrained by its row, its column, its box and both diagonals — five lines in total — so it is one of the most constrained cells on the whole board and a great place to start hunting for a forced digit.
The central 3x3 box is also special: both diagonals pass straight through it, and each diagonal enters through one corner of the box and leaves through the opposite corner. Watching how the two diagonals share the central box often unlocks the puzzle.
How to play online
Choose a difficulty and click any cell to select it. Use the number pad or your keyboard to place digits, turn on Notes to pencil in candidates, and use Auto notes to fill every empty cell with its current candidates so you can focus on the diagonals.
The board highlights the row, column and box of the selected cell, the two diagonals stay shaded, and completed digits dim on the number pad. Your progress saves locally in your browser, so you can step away and return to the same grid.
- Select a cell, then tap a number or press 1–9.
- Use Notes and Auto notes to track candidates, diagonals included.
- Use Hint to reveal one correct cell when stuck.
- Undo and Erase let you walk moves back safely.
- Easy keeps more givens; Expert leans hard on the diagonal logic.
Difficulty levels and why play X-Sudoku
Because the two diagonals add constraints, an X-Sudoku can have a single solution with fewer starting digits than a plain Sudoku. Easy keeps plenty of givens for a relaxed solve, Medium is a balanced daily challenge, and Hard and Expert lean more and more on spotting diagonal singles.
X-Sudoku is the perfect next step for anyone comfortable with classic Sudoku: the rules take seconds to learn but open up fresh deductions, especially around the corners and the centre. Every grid here is checked for one unique solution, so it always rewards logic over guessing.
- Easy: more givens and gentle diagonal scanning.
- Medium: a balanced everyday diagonal Sudoku.
- Hard: fewer givens and more diagonal deduction.
- Expert: the two diagonals carry much of the solve.