Hard Irregular Diagonal Sudoku

Jigsaw regions plus two diagonal constraints — every digit 1–9 must appear once in each row, column, region, and diagonal. Free to play.

Easy Medium Hard Expert

Hard Irregular Diagonal Sudoku. Fewer givens, complex region shapes, and tight diagonal constraints — multi-step pencil-mark work required.

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

What is Irregular Diagonal Sudoku?

Difficulty
★★★★☆
4/5
Constraint Type
Combined: Irregular + Diagonal
Typical Givens
22–30
Avg. Solve (Hard)
35 min

Solving Techniques for Hard Level

Technique Description Level
Map All 29 Units First You have 9 rows + 9 columns + 9 irregular regions + 2 diagonals = 29 constraint units. Identify all irregular regions and both diagonals before starting. Beginner
Diagonal–Region Intersection Cells on a diagonal that also sit in the same irregular region create a powerful two-unit intersection — use both constraints simultaneously. Intermediate
Diagonal Pointing Pairs If a digit on a diagonal is confined to one irregular region, eliminate it from all other cells of that region. Intermediate
Region Coloring for Diagonals Trace which irregular regions each diagonal passes through. Regions with many diagonal cells are the most constrained. Advanced
Chain Deduction Across Units With 29 units, a forced digit in one unit often cascades through 3–4 other units. Track chains carefully to avoid errors. Advanced

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

Average Solve Time by Difficulty

Easy
8 min
Medium
18 min
Hard
35 min
Expert
65 min
Want a full walkthrough of rules, strategies, and solving steps? How to Play Irregular Diagonal Sudoku →

Frequently Asked Questions — Hard Irregular Diagonal Sudoku

What makes hard Irregular Diagonal Sudoku so challenging?
Hard puzzles have fewer givens, more complex region shapes, and require you to chain diagonal, region, and row/column constraints together in long sequences. The interaction between the diagonals and the jigsaw regions creates constraint webs that take sustained, systematic pencil-mark work to unravel.
How should I organise my pencil marks for hard puzzles?
Mark candidates in every empty cell from the start. When you place a digit, immediately update every cell that shares a row, column, region, or diagonal with the placed cell. Hard puzzles have long forcing chains that only become visible when your candidate lists are completely up to date.
What is a multi-constraint forcing chain?
In this variant, a forcing chain can use row, column, region, and diagonal constraints in any order. For example: assuming digit X in cell A eliminates it from a diagonal, which forces digit Y into a region cell, which forces digit Z into a row. Following such chains is the main technique that unlocks hard Irregular Diagonal Sudoku.
Are there cells that are especially powerful to focus on?
Yes — the centre cell (row 5, column 5) sits on both diagonals and one region. Any candidate you eliminate from it affects two diagonals and a region simultaneously. Similarly, corner cells sit on one diagonal and one region; their constraints propagate widely. Focus on these high-constraint cells first.
How long does a hard Irregular Diagonal Sudoku take?
Expect 40–70 minutes for hard puzzles. Most solvers find one or two pivotal cells where a diagonal constraint intersects a region constraint in a non-obvious way — once these are found, the grid unlocks rapidly. Use pencil marks from the first move.

More questions? See the full Irregular Diagonal Sudoku guide.