Hard Diagonal Sudoku

Both main diagonals must also contain 1–9 without repetition. Four units instead of three — twice the constraint density.

Easy Medium Hard Expert

Hard Diagonal Sudoku. Diagonal X-Wings, multi-constraint box intersections, and deep elimination chains — pencil marks are essential.

8
5
9
5
4
6
9
8
3
8
4
1
7
6
7
9
4
3
7
1
2
3
5
2
7
4
9
Mistakes
0/3
Score
-
Time
00:00
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Progress0%

What is Diagonal Sudoku?

Difficulty
★★☆☆☆
2/5
Constraint Type
Extra Regions
Typical Givens
22–28
Avg. Solve (Hard)
20 min

Solving Techniques for Hard Level

Technique Description Level
Diagonal as a Unit Treat each diagonal exactly like a row or column — every digit 1–9 appears exactly once. Standard scanning and counting apply. Beginner
Diagonal–Box Interaction When a digit on a diagonal is confined to one box segment, eliminate it from the rest of that box. Intermediate
Diagonal Naked Pairs If two cells on a diagonal share the same two candidates, eliminate those values from all other diagonal cells. Intermediate
Three-Unit Intersection A cell on the main diagonal also belongs to a row, column, and box — four constraint units simultaneously — maximising elimination power. Advanced

Ready for the ultimate test? Try Expert Diagonal Sudoku — the hardest puzzles on Sudoku.by.

Average Solve Time by Difficulty

Easy
4 min
Medium
10 min
Hard
20 min
Expert
40 min
Want a full walkthrough of rules, strategies, and solving steps? How to Play Diagonal Sudoku →

Frequently Asked Questions — Hard Diagonal Sudoku

What makes hard Diagonal Sudoku difficult?
Hard puzzles have fewer given digits and require chaining diagonal eliminations with box, row, and column techniques. You'll need to apply hidden pairs and triples within the diagonals, and exploit the intersection of the diagonal with box constraints for multi-step deductions.
How do diagonal X-Wings work in hard Diagonal Sudoku?
A diagonal X-Wing occurs when a digit appears in only two positions on each of two rows (or columns), and those positions both lie on the same diagonal. The diagonal constraint then eliminates that digit from all other cells on the diagonal, often cracking a previously stuck position.
Should I use pencil marks for hard Diagonal Sudoku?
Absolutely. Hard puzzles require tracking candidates across rows, columns, boxes, and both diagonals simultaneously. Without pencil marks, the five-way constraint interactions are too complex to hold in memory.
What is the trickiest aspect of hard Diagonal Sudoku?
The hardest deductions involve cells that sit on a diagonal AND at the intersection of two boxes. These cells carry constraints from up to five groups at once (row, column, two boxes, diagonal), and identifying a forced placement requires considering all five simultaneously.
How long does a hard Diagonal Sudoku take?
Expect 25–50 minutes. If you're stuck, focus on the center box — both diagonals pass through it, making it the most constrained box on the board and usually the key to the next breakthrough.

More questions? See the full Diagonal Sudoku guide.