What is Sudoku X?
Sudoku X (also called Diagonal Sudoku or X Sudoku) is a variant of classic Sudoku that adds a single diagonal constraint. The grid follows all standard Sudoku rules—every row, column, and 3×3 box must contain the digits 1-9 without repetition—plus both long diagonals must each contain 1-9 exactly once.
The two diagonals are the main diagonal (top-left to bottom-right, cells R1C1 through R9C9) and the anti-diagonal (top-right to bottom-left, cells R1C9 through R9C1). Cells on these diagonals have stricter placement rules than off-diagonal cells because they must satisfy an additional uniqueness constraint. The center cell (R5C5) is the only cell on both diagonals simultaneously. It has five uniqueness constraints instead of the usual four: its row, column, box, plus both diagonals.
Every puzzle has exactly one solution. PencilGrid's generator guarantees unique solutions. Difficulty ratings are based on clue count: Easy puzzles start with 32 given digits, Medium with 26, and Hard with 22. The daily difficulty rotates throughout the week. Practice mode offers unlimited puzzles at your chosen difficulty.
Features
Daily Puzzles
New Sudoku X every day at midnight UTC. Build a streak by solving consecutive days.
Practice Mode
Unlimited puzzles at Easy, Medium, and Hard difficulty. Perfect for learning diagonal elimination logic.
Diagonal Highlighting
Diagonal cells are visually highlighted on the board so you can see which cells have the extra constraint.
Notes Mode
Pencil in candidate numbers (notes) in each cell. Essential for tracking diagonal eliminations.
Error Checking
Check your work at any time. Conflicts in rows, columns, boxes, and diagonals are highlighted.
Undo/Redo
Full undo/redo stack. Experiment with diagonal placements freely without fear of losing progress.
How to Play
The Rules
1.
Fill the grid with digits 1-9 following standard Sudoku rules: each row, column, and 3×3 box must contain all digits without repetition.
2.
Both long diagonals must also each contain the digits 1-9 exactly once. The main diagonal runs from top-left to bottom-right. The anti-diagonal runs from top-right to bottom-left.
3.
Cells on the diagonals have five uniqueness constraints (row, column, box, and two diagonals if on both). The center cell R5C5 is the only cell on both diagonals.
4.
Diagonal cells are highlighted with a subtle background tint on the board so you can see which cells have the extra constraint.
Basic Strategy
Start by scanning the diagonals. If a digit already appears on a diagonal, it's ruled out for every other cell on that diagonal. Example: if R1C1=6, R2C2=2, R3C3=8, and R8C8=9 are filled on the main diagonal, the center cell R5C5 cannot be any of {2,6,8,9}—leaving only {1,3,4,5,7} as candidates, even if its row, column, and box would otherwise allow all nine digits. Use notes mode to track diagonal eliminations.
Frequently Asked Questions
What is the difference between Sudoku X and regular Sudoku?
Sudoku X follows all the rules of classic Sudoku—every row, column, and 3×3 box must contain the digits 1–9 without repetition. The single difference is that both long diagonals (top-left to bottom-right and top-right to bottom-left) must also each contain 1–9 exactly once. That diagonal constraint is the entire difference between the two puzzles.
Is Sudoku X harder than classic Sudoku?
Not inherently. The diagonals add constraints that both create new deductions and add a dimension to track. On PencilGrid, difficulty is clue count: Easy has 32 given digits, Medium has 26, Hard has 22. The diagonals change placement logic, but difficulty comes from how few clues you start with.
Do both diagonals need 1–9, or just one?
Both. The main diagonal (top-left to bottom-right) and the anti-diagonal (top-right to bottom-left) must each contain the digits 1–9 exactly once. Both diagonals are active constraints in every Sudoku X puzzle.
What if a cell is on both diagonals?
Exactly one cell—the center cell at R5C5—lies on both diagonals. It must satisfy both diagonal constraints simultaneously, on top of its row, column, and 3×3 box rules. It's the only cell in the grid with five uniqueness constraints instead of four.
Can I play Sudoku X on mobile?
Yes. The board is fully responsive and works on phones and tablets. All controls (digit entry, notes mode, undo/redo, checking) are touch-friendly. Your progress saves automatically, so you can pause mid-puzzle and resume later on any device.
Explore More
How to Play Sudoku X
Learn the diagonal rule with a worked example showing how diagonal elimination differs from standard Sudoku logic.
Daily Puzzle Info
A fresh Sudoku X puzzle every day at midnight UTC. Difficulty rotates by weekday across Easy, Medium, and Hard.
Play Today's Puzzle
Jump straight into today's daily Sudoku X board and start solving with diagonal constraints.
Practice Mode
Unlimited Sudoku X puzzles at Easy (32 clues), Medium (26 clues), and Hard (22 clues) difficulty levels.
Play Classic Sudoku
Try the traditional 9×9 Sudoku with given digits and no diagonal constraints.
Play Killer Sudoku
Solve Sudoku with cage sum constraints instead of given digits.