Diagonal Sudoku is standard 9×9 Sudoku with one additional condition: the two long corner-to-corner diagonals must also contain digits 1–9 without repetition. Those diagonals are extra units, not replacements for boxes.
How does Diagonal Sudoku work?
In Diagonal Sudoku, every row, column and 3×3 box contains digits 1–9 once, and both main diagonals also contain digits 1–9 once. Treat each diagonal like another nine-cell unit. Use it to remove candidates, find hidden singles and create intersections, while remembering that short partial diagonals have no special rule.
What are the rules of Diagonal Sudoku?
Begin with all classic rules. A digit cannot repeat in a row, column or 3×3 box. Then inspect the diagonal running from top-left to bottom-right and the diagonal running from top-right to bottom-left. Each must also contain digits 1–9 exactly once.
Only the two full nine-cell diagonals receive the extra rule unless the puzzle states otherwise. A group of cells that merely forms a shorter diagonal inside a box is not a unit.
What is the best strategy for Diagonal Sudoku?
Mark diagonal membership clearly, then begin with ordinary singles. For each diagonal, list missing digits and eliminate positions through their crossing rows, columns and boxes. A digit may be legal by classic rules but impossible because it already appears elsewhere on the same main diagonal.
Scan the interaction in both directions. A diagonal hidden single can place a digit that simplifies a box; a box placement can leave one position for a digit on a diagonal. Recheck both diagonals after every placement touching either one.
Can a diagonal create locked candidates?
Yes. If all candidates for 6 on one diagonal lie inside the same 3×3 box, that box must place its diagonal 6 in one of those cells. Candidate 6 can be removed from other diagonal cells only through the diagonal rule, and ordinary box candidates may be reduced when the overlap proves a position set.
Why is the center cell important?
The center cell belongs to its row, column, box and both diagonals—five units in total. That does not guarantee it is solved first, but it receives more exclusions than any other cell and often becomes restricted early.
Do not assume the center has a special fixed value. Its digit depends on the puzzle. Calculate candidates from all five units rather than relying on symmetry.
What mistakes are common in Diagonal Sudoku?
- Forgetting to remove a digit already used on a main diagonal.
- Applying the diagonal rule to short decorative diagonals.
- Ignoring classic boxes while concentrating on the X shape.
- Assuming diagonal positions must be symmetrical.
- Using a standard Sudoku solver that does not support diagonal constraints.
A useful solving loop is classic units, first diagonal, second diagonal, then classic units again. The extra rules should simplify the puzzle, not replace disciplined candidate checks.
Our game / Story-led deduction
Want each placement to reveal part of a mystery?
Detective Sudoku is not classic Sudoku. It replaces digits and boxes with suspects, rooms and truthful spatial clues while retaining one suspect per row and column. Its real hints identify the clue behind a deduction instead of giving you the answer or solution.
See what makes Detective Sudoku different →Frequently asked questions about Diagonal Sudoku
Is Diagonal Sudoku harder than normal Sudoku?
It has extra constraints, but those constraints can create more forced moves. Difficulty depends on the construction.
Does every diagonal contain 1 through 9?
Only the two full main diagonals, unless the puzzle explicitly marks additional regions.
Can the same digit appear on both diagonals?
Yes, as long as it appears once on each and does not violate crossing row, column or box rules.
Is the center digit shared by both diagonals?
Yes. The center is the one cell belonging to both diagonal units.