Expert 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

Expert Diagonal Sudoku. The hardest puzzles on the site — five constraint groups per cell with very few givens. Every placement follows from precise logic. No guessing.

9
5
1
8
3
6
6
1
4
8
3
1
4
5
8
2
6
7
5
7
1
3
7
3
2
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 (Expert)
40 min

Solving Techniques for Expert 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

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 — Expert Diagonal Sudoku

Is expert Diagonal Sudoku the hardest level on Sudoku.by?
Expert Diagonal Sudoku requires mastery of both standard advanced techniques and diagonal-specific logic. With very few given digits, the solve demands multi-step reasoning that treats the two diagonals as first-class constraint groups alongside rows, columns, and boxes.
Can you solve expert Diagonal Sudoku by guessing?
No. Every expert puzzle has a unique logical solution. The five-way constraint structure (rows, columns, boxes, main diagonal, anti-diagonal) provides enough information for a pure-logic solve, but that logic can be deeply nested and non-obvious.
What advanced strategies work best for expert Diagonal Sudoku?
Use 'diagonal coloring': assign candidate digits alternative colors along a diagonal chain and check for color conflicts. Also look for Swordfish patterns that span both a diagonal and multiple rows — these are rare but very powerful when they appear.
How does the anti-diagonal interact with the main diagonal at expert level?
Both diagonals share only the center cell (5,5). But they both pass through the same three boxes (top-left, center, bottom-right for the main diagonal; top-right, center, bottom-left for the anti-diagonal). Using eliminations from both diagonals within these boxes simultaneously is the hallmark of expert-level diagonal technique.
How long does an expert Diagonal Sudoku take?
Expert puzzles typically take 50–80 minutes. If progress stalls, revisit every cell on both diagonals and recheck your candidate lists — it's common to miss a diagonal hidden single that has been enabled by recent placements elsewhere on the board.

More questions? See the full Diagonal Sudoku guide.