RULES & STRATEGY

Diagonal Sudoku (Sudoku X).

Diagonal Sudoku, also called Sudoku X, adds two groups to the classic 9×9 puzzle. The two shaded corner-to-corner diagonals must each contain 1–9 exactly once.

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THE OBJECTIVE

How to play

Fill every empty square with a number from 1 to 9. Each row, column and 3 × 3 box must contain every number exactly once. Both shaded main diagonals must also contain 1–9 exactly once.

Use the numbers 1–9, with no repeats in any row or column.
Each outlined 3 × 3 box also needs every number exactly once. Printed starting numbers cannot be changed.
Both shaded corner-to-corner diagonals must contain 1–9 without repeats. The center square belongs to both diagonals.
Every puzzle has exactly one solution. Repeated numbers are marked with an ×. Hint reveals a correct number; Undo restores the previous move, including notes.

A rule in action

The shaded main diagonal already has 1, 2, 3, 4, 6, 7, 8 and 9. Its empty center must be 5. This is a teaching fragment, not a full puzzle.

Keep all the classic rules

Rows, columns and 3×3 boxes still need 1–9 without repeats. The diagonal rules are additional constraints, not replacements. A number placed outside the shaded diagonals follows only the usual row, column and box rules.

Follow only the two main diagonals

The shaded line from the top-left to bottom-right corner is one group. The line from top-right to bottom-left is the other. Other sloping lines do not receive an extra rule. The center square belongs to both diagonals, so its number affects both groups at once.

Use diagonal intersections to eliminate candidates

A shaded square must satisfy its row, column, box and diagonal. The center must satisfy both diagonals too. If a main diagonal has eight distinct values, the remaining one is forced. Use Notes to record candidates after checking every group. Duplicate marks include diagonal conflicts; an unmarked value still is not proof of a correct solution.

STEP BY STEP

Solve a complete example

Start with the grid on the left. Find the allowed numbers for each empty square. This walkthrough uses only single candidates and numbers with just one possible place in a group — no guessing.

Both shaded diagonals are extra groups. Number the boxes from left to right, top to bottom; diagonal 1 runs from top left to bottom right.

Your starting grid

8	5	·	·	6	3	1	2	9
9	·	2	·	8	·	·	·	·
1	7	3	2	·	·	·	6	·
·	4	8	·	·	6	·	·	·
6	·	5	9	7	·	2	4	·
2	·	·	3	·	·	7	·	·
·	·	·	·	·	·	5	·	8
5	·	·	8	·	·	4	9	3
·	·	·	·	3	·	6	·	·

Show the complete solution

8	5	4	7	6	3	1	2	9
9	6	2	4	8	1	3	5	7
1	7	3	2	9	5	8	6	4
7	4	8	1	2	6	9	3	5
6	3	5	9	7	8	2	4	1
2	1	9	3	5	4	7	8	6
3	9	1	6	4	2	5	7	8
5	2	6	8	1	7	4	9	3
4	8	7	5	3	9	6	1	2

All 45 steps explained

Row 1, column 3: 4. Checking every group containing this square leaves this as its only candidate.
Row 1, column 4: 7. Checking every group containing this square leaves this as its only candidate.
Row 2, column 2: 6. Checking every group containing this square leaves this as its only candidate.
Row 2, column 7: 3. Checking every group containing this square leaves this as its only candidate.
Row 2, column 8: 5. Checking every group containing this square leaves this as its only candidate.
Row 3, column 7: 8. Checking every group containing this square leaves this as its only candidate.
Row 3, column 9: 4. Checking every group containing this square leaves this as its only candidate.
Row 2, column 9: 7. Checking every group containing this square leaves this as its only candidate.
Row 4, column 4: 1. Checking every group containing this square leaves this as its only candidate.
Row 2, column 4: 4. Checking every group containing this square leaves this as its only candidate.
Row 2, column 6: 1. Checking every group containing this square leaves this as its only candidate.
Row 4, column 7: 9. Checking every group containing this square leaves this as its only candidate.
Row 4, column 8: 3. Checking every group containing this square leaves this as its only candidate.
Row 4, column 1: 7. Checking every group containing this square leaves this as its only candidate.
Row 4, column 9: 5. Checking every group containing this square leaves this as its only candidate.
Row 4, column 5: 2. Checking every group containing this square leaves this as its only candidate.
Row 5, column 6: 8. Checking every group containing this square leaves this as its only candidate.
Row 5, column 9: 1. Checking every group containing this square leaves this as its only candidate.
Row 5, column 2: 3. Checking every group containing this square leaves this as its only candidate.
Row 6, column 6: 4. Checking every group containing this square leaves this as its only candidate.
Row 6, column 5: 5. Checking every group containing this square leaves this as its only candidate.
Row 3, column 5: 9. Checking every group containing this square leaves this as its only candidate.
Row 3, column 6: 5. Checking every group containing this square leaves this as its only candidate.
Row 6, column 8: 8. Checking every group containing this square leaves this as its only candidate.
Row 6, column 9: 6. Checking every group containing this square leaves this as its only candidate.
Row 7, column 3: 1. Checking every group containing this square leaves this as its only candidate.
Row 6, column 3: 9. Checking every group containing this square leaves this as its only candidate.
Row 6, column 2: 1. Checking every group containing this square leaves this as its only candidate.
Row 7, column 4: 6. Checking every group containing this square leaves this as its only candidate.
Row 7, column 5: 4. Checking every group containing this square leaves this as its only candidate.
Row 7, column 1: 3. Checking every group containing this square leaves this as its only candidate.
Row 7, column 8: 7. Checking every group containing this square leaves this as its only candidate.
Row 8, column 2: 2. Checking every group containing this square leaves this as its only candidate.
Row 7, column 2: 9. Checking every group containing this square leaves this as its only candidate.
Row 7, column 6: 2. Checking every group containing this square leaves this as its only candidate.
Row 8, column 5: 1. Checking every group containing this square leaves this as its only candidate.
Row 8, column 6: 7. Checking every group containing this square leaves this as its only candidate.
Row 8, column 3: 6. Checking every group containing this square leaves this as its only candidate.
Row 9, column 1: 4. Checking every group containing this square leaves this as its only candidate.
Row 9, column 2: 8. Checking every group containing this square leaves this as its only candidate.
Row 9, column 3: 7. Checking every group containing this square leaves this as its only candidate.
Row 9, column 4: 5. Checking every group containing this square leaves this as its only candidate.
Row 9, column 6: 9. Checking every group containing this square leaves this as its only candidate.
Row 9, column 8: 1. Checking every group containing this square leaves this as its only candidate.
Row 9, column 9: 2. Checking every group containing this square leaves this as its only candidate.

Common questions

Do all diagonal lines need unique numbers?

No. Only the two shaded main diagonals that connect opposite corners have the extra rule.

Why is the center square special?

It is part of both main diagonals, as well as its row, column and box. All of those groups constrain its value.

Controls, saves and offline play

Select a square and choose a number. Notes mark candidates, and Erase clears an entry. Arrow keys move your selection; number keys enter values. N toggles notes, and Backspace or Delete erases.

H gives a hint, Ctrl/⌘ Z undoes a move, and F toggles fullscreen where supported. Each variant saves separately on this device. Switching language keeps the same save. Up to 100 undo steps are retained.

First open the game online. Once the menu says “Available offline”, you can reopen it without a connection while your browser retains its data. Clearing site data also removes saved games.

Try it at the table.

Play Diagonal Sudoku (Sudoku X) →