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Sudoku X - Free Diagonal Sudoku Puzzle

Sudoku X takes the classic 9x9 Sudoku puzzle and adds a compelling twist: both main diagonals must also contain the numbers 1 through 9 without repetition. That means every row, every column, every 3x3 box, and each of the two diagonals stretching from corner to corner must each hold all nine digits exactly once. This single extra constraint transforms familiar solving strategies and opens up entirely new lines of logic.

The diagonal rule might sound like a small addition, but it has a big impact on how you approach each puzzle. Cells that sit on a diagonal are now constrained by four groups instead of the usual three, which means you can often narrow down their candidates much faster. At the same time, the interplay between diagonal constraints and box boundaries creates patterns you will not find in standard Sudoku.

Whether you are a seasoned Sudoku solver looking for a fresh challenge or a curious beginner ready to explore beyond the classic format, Sudoku X on Mini Puzzles is the perfect place to sharpen your logic skills. Play a new puzzle every day, tackle unlimited random grids, or use the solver to study advanced techniques — all free, right in your browser.

How to Play Sudoku X

Sudoku X follows all the standard Sudoku rules with one powerful addition: both main diagonals must also contain the numbers 1 through 9 exactly once. This extra constraint gives you more information to work with on every puzzle.

If you already know how to solve regular Sudoku, you are ready for Sudoku X. The diagonal rule layers naturally on top of existing logic, and once you start using it, you will wonder how you ever solved without it.

Rules

  • 1

    Fill the 9x9 Grid

    Place the numbers 1 through 9 in every empty cell. Each row must contain all nine digits without repeating. Each column must also contain all nine digits without repeating. Each of the nine 3x3 boxes must contain all nine digits without repeating.

  • 2

    Satisfy Both Diagonals

    The main diagonal running from the top-left corner to the bottom-right corner must contain the numbers 1 through 9 exactly once. The other main diagonal, running from the top-right corner to the bottom-left corner, must also contain 1 through 9 exactly once. These two diagonals form an "X" across the grid, which gives the puzzle its name.

  • 3

    Start from the Clues

    Some cells are pre-filled with numbers. These are your starting clues. Use logic to determine which numbers belong in the remaining cells. Every Sudoku X puzzle on Mini Puzzles has exactly one valid solution.

  • 4

    Solve with Pure Logic

    You never need to guess. Every cell can be deduced through logical reasoning alone. If you feel stuck, look for cells on a diagonal — they belong to four constraint groups at once, which often reveals the answer.

Tips for Using the Diagonals

Start at the Center

The center cell (row 5, column 5) belongs to a row, a column, a box, and both diagonals — five groups in total. That makes it the most constrained cell on the board and often one of the easiest to solve first.

Scan the Diagonals Early

Before diving into rows and columns, scan both diagonals for given numbers. Each diagonal only has nine cells, so even a few clues can dramatically limit what goes where. Treat the diagonals like bonus rows.

Corner Cells Are Key

The four corner cells of the grid each sit on a diagonal, a row, a column, and a box — four constraint groups. When you place a number in a corner, it eliminates that number from an entire diagonal, opening up deductions across the board.

Use Diagonal Elimination

When a number appears on a diagonal, you can immediately rule it out for every other cell on that diagonal. Combine this with row, column, and box elimination to resolve cells that would be ambiguous in standard Sudoku.

Note the Overlap Cells

Some cells sit at the intersection of a diagonal and a box boundary, giving them extra constraints. Pay special attention to where the diagonals cross through each 3x3 box — these overlap zones are where breakthroughs happen most often.

Use Notes Generously

Toggle notes mode (the pencil icon) to mark candidate numbers in each cell. In Sudoku X, candidates on diagonal cells get eliminated faster, so keeping pencil marks up to date will help you spot naked singles and hidden singles quickly.

Frequently Asked Questions

What is Sudoku X?

Sudoku X, also known as Diagonal Sudoku, is a variant of the classic 9x9 Sudoku puzzle. It follows all the standard rules — every row, column, and 3x3 box must contain the numbers 1 through 9 exactly once — but adds one extra constraint: the two main diagonals of the grid must also contain 1 through 9 without repetition. The diagonals form an "X" shape across the board, which is where the name comes from. This additional rule makes the puzzle both more constrained and more interesting to solve.

How do the diagonals change solving strategy?

In standard Sudoku, each cell belongs to three groups: a row, a column, and a box. In Sudoku X, cells on the main diagonals belong to a fourth group (or even a fifth for the center cell, which sits on both diagonals). This extra membership means those cells have more constraints, so you can often eliminate candidates faster. You will find that starting your solve by scanning the diagonals — much like scanning a row or column — frequently reveals placements that are impossible to find with standard techniques alone. Strategies like X-Wing and Swordfish can also interact with diagonal constraints in unique ways.

Is Sudoku X harder than regular Sudoku?

It depends on how you look at it. The diagonal constraint actually makes the puzzle more restricted, which means there are fewer valid solutions and often fewer possible candidates per cell. In that sense, the extra rule gives you more tools to work with and can make certain deductions easier. However, Sudoku X puzzles are typically designed with fewer starting clues to compensate, and you need to keep track of the diagonals on top of everything else. Most players find that Sudoku X feels slightly more challenging at first, but becomes very satisfying once you learn to use the diagonals to your advantage.

What tips do you have for Sudoku X beginners?

Here are a few practical tips to get started:

  • • Solve the center cell first. It belongs to five groups (row, column, box, and both diagonals), so it is usually the most constrained cell on the board.
  • • Treat each diagonal like a row. After filling in a few diagonal cells, scan for missing numbers the same way you would scan a row or column.
  • • Use notes mode liberally. Pencil marks on diagonal cells get pruned faster than other cells, so keeping them current helps you spot singles quickly.
  • • Start with an easier difficulty. The diagonal rule takes some getting used to, so play a few easy puzzles before moving up.
Why is the center cell so important in Sudoku X?

The center cell of the grid (row 5, column 5) is unique because it is the only cell that lies on both main diagonals. That means it is simultaneously constrained by its row, its column, the center 3x3 box, and both diagonals — five groups in total. In a standard Sudoku, each cell is constrained by only three groups. This makes the center cell the most restricted position on the entire board, and it is often one of the first cells you can solve. Experienced Sudoku X players almost always check the center cell early in their solve.

Is Sudoku X on Mini Puzzles free to play?

Yes, Sudoku X is completely free on Mini Puzzles. There are no ads, no subscriptions, and no account required. You can play the daily puzzle, generate unlimited random puzzles, or use the solver — all from any modern browser on desktop, tablet, or phone. Your progress and best times are saved locally on your device so you can track your improvement over time.

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