Diagonal Sudoku - commonly known as Sudoku X - is one of the most elegant and accessible Sudoku variants. By adding just two extra constraints (the main diagonals), it transforms the classic puzzle into something that feels both familiar and refreshingly new. The X-shaped diagonal requirement creates unique solving paths and makes every puzzle feel distinctly different from standard Sudoku.
Diagonal Sudoku uses the same 9x9 grid and 3x3 boxes as classic Sudoku. The twist is simple but powerful: in addition to the standard rules, both main diagonals of the grid must also contain the digits 1 through 9 without repetition.
The two diagonals form an X pattern across the grid - hence the alternate name "Sudoku X." The top-left to bottom-right diagonal and the top-right to bottom-left diagonal each become an additional "unit" that must satisfy the same uniqueness constraint as rows, columns, and boxes.
This seemingly small addition has a significant impact on solving. Cells along the diagonals have an extra constraint to satisfy, which means they can often be solved earlier than in classic Sudoku. The center cell is particularly special - it sits at the intersection of both diagonals, the center row, the center column, and the center box, making it the most constrained cell in the entire grid.
Diagonal Sudoku follows all standard Sudoku rules, plus two additional constraints: the main diagonal from the top-left corner to the bottom-right corner must contain digits 1 through 9 exactly once, and the main diagonal from the top-right corner to the bottom-left corner must also contain digits 1 through 9 exactly once. All standard row, column, and 3x3 box rules remain in effect.
The diagonals are treated as two additional "units" alongside the 9 rows, 9 columns, and 9 boxes. This means cells on the diagonals must satisfy four constraints (row, column, box, and diagonal) instead of the usual three.
Diagonal Sudoku is rated as intermediate difficulty. Interestingly, the extra diagonal constraints can make puzzles easier to begin because diagonal cells have more restrictions, providing additional entry points for solving. However, puzzle designers can create very challenging Diagonal Sudoku puzzles by exploiting the interactions between diagonal and box constraints.
This variant is ideal for solvers who have mastered basic Sudoku and want a gentle step up in difficulty. It is the perfect first variant to try because the core solving experience remains very close to classic Sudoku - you are simply adding two more lines to check. If you are looking for your first Sudoku variant, Diagonal Sudoku is an excellent choice.
Start with the center cell. Because it sits on both diagonals plus the center row, column, and box, it has the most constraints of any cell in the grid. Check what digits are already present in all five of its units - often you can solve it very early.
Next, focus on cells where the diagonals intersect with corners and edges of 3x3 boxes. These intersections create powerful elimination opportunities. For example, if a digit appears in a box that a diagonal passes through, it is eliminated from the diagonal cells in that box.
Use all your standard Sudoku solving strategies - naked singles, hidden singles, pairs, and pointing pairs - but remember to include the diagonals as additional units when scanning for eliminations. Many solvers forget to check the diagonals and miss easy deductions.
Diagonal Sudoku is one of the oldest Sudoku variants, with roots stretching back to the earliest days of number placement puzzles. The concept of adding diagonal constraints to a Latin square predates modern Sudoku by decades - diagonal Latin squares were studied by mathematicians in the 18th and 19th centuries.
When Sudoku became a global phenomenon in the mid-2000s, Diagonal Sudoku was among the first variants to appear in newspapers and puzzle books. Its simplicity made it easy to explain to a mass audience already familiar with classic Sudoku. The variant is commonly featured in the World Sudoku Championship and remains one of the most popular Sudoku variants worldwide due to its elegant balance of simplicity and depth.
Diagonal Sudoku, also called Sudoku X, adds two extra constraints to classic Sudoku. In addition to the standard row, column, and box rules, both main diagonals - from the top-left corner to the bottom-right corner, and from the top-right corner to the bottom-left corner - must also contain the digits 1 through 9 exactly once.
Diagonal Sudoku can actually be slightly easier to start than classic Sudoku because the diagonal constraints provide additional information for elimination. The center cell, which sits on both diagonals, is especially constrained. However, harder Diagonal Sudoku puzzles can be quite challenging as they exploit the diagonal interactions in complex ways.
No. The diagonal constraints are added on top of all standard Sudoku rules. Every row, column, and 3x3 box must still contain digits 1 through 9 exactly once. The diagonals are an additional requirement, not a replacement.
The center cell (row 5, column 5) lies on both main diagonals, in the center box, and in the middle row and column. This means it is constrained by four units instead of the usual three, making it one of the most restricted cells in the entire grid and often one of the easiest to solve early.
Diagonal Sudoku puzzles appear in many puzzle books, newspaper supplements, and online puzzle platforms. They are also featured regularly in the World Sudoku Championship and other competitive events organized by the World Puzzle Federation.
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