What is the difference between Diagonal Sudoku and regular Sudoku?

Diagonal Sudoku adds one extra rule on top of Classic Sudoku: both main diagonals of the 9x9 grid must also contain the digits 1 through 9 exactly once. This additional constraint means that each of the 18 diagonal cells has one more group it must satisfy, which makes the puzzle logic tighter and often allows fewer starting clues.

Is Diagonal Sudoku harder than Classic Sudoku?

Not necessarily. The diagonal constraint provides extra information, which can actually make some puzzles easier to solve because you have more ways to eliminate candidates. However, puzzle designers often compensate by providing fewer starting clues, which can make Diagonal Sudoku feel more challenging. The difficulty depends on the specific puzzle, not the variant itself.

Why is the center cell so important in Diagonal Sudoku?

The center cell (row 5, column 5) is the only cell in the entire grid that belongs to five constraint groups: its row, its column, the center 3x3 box, the main diagonal, and the anti-diagonal. This makes it the most constrained cell on the board, and determining its value early can unlock a cascade of deductions across the grid.

Can I use standard Sudoku techniques to solve Diagonal Sudoku?

Yes. Every standard Sudoku technique — naked singles, hidden singles, pointing pairs, box-line reduction, X-wings, and more — works in Diagonal Sudoku. You simply treat each diagonal as an additional group (like an extra row or column) when scanning for eliminations. This actually gives you more opportunities to apply those techniques.

Where can I play Diagonal Sudoku online or on my phone?

You can play Diagonal Sudoku in the Sudoku - Brain Puzzles app, available for free on iOS. The app includes Diagonal Sudoku along with seven other variants including Classic, Killer, Sandwich, Little Killer, Mini, Mini Sandwich, and Mega 16x16.

Diagonal Sudoku Rules: How to Play & Strategy Tips

Diagonal Sudoku, also known as Sudoku X, is a variant that takes the familiar 9x9 grid and adds one powerful twist: both main diagonals must also contain the digits 1 through 9 exactly once. This single extra rule changes the way you think about candidate elimination and opens up new solving strategies that do not exist in Classic Sudoku.

What Is Diagonal Sudoku?

In a standard Sudoku puzzle, you fill a 9x9 grid so that every row, every column, and every 3x3 box contains all digits from 1 to 9 without repetition. Diagonal Sudoku keeps all of those rules and layers on one more: the two main diagonals of the grid are treated as additional groups, just like rows and columns.

The main diagonal runs from the top-left corner (row 1, column 1) to the bottom-right corner (row 9, column 9). The anti-diagonal runs from the top-right corner (row 1, column 9) to the bottom-left corner (row 9, column 1). Each of these diagonals contains exactly nine cells, and each must hold the digits 1 through 9 with no duplicates.

You may see this variant called Sudoku X because the two diagonals form an X shape across the grid. The names Diagonal Sudoku and Sudoku X are interchangeable -- they describe the same rule set.

Rules of Diagonal Sudoku

Diagonal Sudoku uses four rules in total. The first three are identical to Classic Sudoku, and the fourth is the diagonal constraint:

Row rule: Each row must contain the digits 1 through 9 exactly once. No digit may repeat within a single row.
Column rule: Each column must contain the digits 1 through 9 exactly once. No digit may repeat within a single column.
Box rule: Each 3x3 box (there are nine of them arranged in a 3x3 pattern) must contain the digits 1 through 9 exactly once.
Diagonal rule: Both the main diagonal (top-left to bottom-right) and the anti-diagonal (top-right to bottom-left) must contain the digits 1 through 9 exactly once. No digit may repeat on either diagonal.

A valid Diagonal Sudoku solution satisfies all four rules simultaneously. If even one digit is repeated on a diagonal, the solution is invalid, regardless of whether the rows, columns, and boxes are correct.

Strategy Tips for Diagonal Sudoku

Because the diagonals act as two extra constraint groups, you have more information to work with when eliminating candidates. Here are the most effective strategies for Diagonal Sudoku.

1. Start with the Center Cell

The center cell of the grid (row 5, column 5) is uniquely powerful in Diagonal Sudoku. It belongs to five constraint groups: row 5, column 5, the center 3x3 box, the main diagonal, and the anti-diagonal. In Classic Sudoku, it belongs to only three groups. Those extra two constraints mean the center cell has the fewest possible candidates of any cell on the board in most puzzles. Determine its value as early as you can -- it often triggers a chain of deductions.

2. Scan the Diagonals for Hidden Singles

Just as you scan rows, columns, and boxes for hidden singles in Classic Sudoku, do the same for each diagonal. If a particular digit can only go in one cell along a diagonal, that cell must contain that digit. Because the diagonals cross through multiple boxes, a hidden single on a diagonal can resolve ambiguities that would be impossible to break using box logic alone.

3. Use Cross-Diagonal Elimination

When you place a digit on one diagonal, check whether the same digit appears as a candidate in cells on the other diagonal that share the same row, column, or box. The intersection of diagonal constraints with standard constraints creates powerful elimination opportunities. For example, if the digit 7 is placed at (2, 2) on the main diagonal, you can eliminate 7 from all other cells in row 2, column 2, the top-left box, and every remaining cell on the main diagonal.

4. Pay Attention to Corner and Edge Diagonal Cells

The four corner cells of the grid each sit on one diagonal, a row, a column, and a box -- that is four constraint groups. Corner cells in Diagonal Sudoku are more constrained than in Classic Sudoku (where they belong to only three groups). Similarly, the cells along the diagonals that fall on box borders (such as positions (1,1), (3,3), (4,4), (6,6), (7,7), (9,9) on the main diagonal) can be especially revealing because they bridge two different 3x3 boxes.

Key strategy: Always check the diagonals after every digit you place. In Classic Sudoku, you check only the affected row, column, and box. In Diagonal Sudoku, if the placed digit sits on a diagonal, you must also eliminate that digit from every other unsolved cell on that diagonal. Forgetting this extra check is the most common mistake players make when switching from Classic to Diagonal Sudoku.

5. Apply Pointing Pairs on Diagonals

A pointing pair occurs when a candidate digit in a box is confined to cells that all lie on the same diagonal. When this happens, you can eliminate that candidate from all other cells on that diagonal outside the box. This technique is the diagonal equivalent of the standard pointing pair strategy used with rows and columns, and it arises frequently because each diagonal passes through three boxes.

6. Leverage the Diagonal for Box-Line Reduction

Box-line reduction works in reverse as well: if a candidate on a diagonal is confined to a single box, then that candidate can be removed from all other cells in that box that are not on the diagonal. This is particularly useful in the three boxes along each diagonal (the corner boxes and the center box), where diagonal cells make up one-third of the box.

How Diagonals Change the Game

Adding the diagonal constraint fundamentally alters the solving experience in several ways.

Fewer starting clues are needed. Because the solver has more constraints to work with, puzzle designers can provide fewer given digits while still ensuring a unique solution. A typical Classic Sudoku puzzle might provide 24 to 30 clues, while a Diagonal Sudoku puzzle of comparable difficulty might start with only 18 to 24 clues. The grid looks emptier, but the extra logic compensates.

The center box becomes critical. Five of the nine cells in the center 3x3 box lie on at least one diagonal: the four corners of the box sit on one diagonal each, and the center cell sits on both. This makes the center box the most interconnected region of the grid. Solving it early can cascade into progress across the entire puzzle.

Standard techniques become more powerful. Techniques like naked pairs, hidden triples, and X-wings still apply, but they interact with the diagonal groups to create additional elimination opportunities. An X-wing pattern that involves diagonal cells, for instance, can eliminate candidates from the diagonal as well as from rows and columns, compounding the effect.

Puzzle symmetry may differ. Classic Sudoku puzzles are often rotationally symmetric -- the pattern of given digits looks the same when the grid is rotated 180 degrees. Diagonal Sudoku puzzles sometimes use reflective symmetry along one or both diagonals instead, since the diagonals are structurally significant.

Whether you are a Classic Sudoku veteran looking for a new challenge or a puzzle enthusiast exploring variants for the first time, Diagonal Sudoku offers a satisfying step up in complexity without requiring entirely new techniques. The rules are simple to learn, and the strategic depth rewards careful, methodical solving.

Diagonal Sudoku Rules