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Sudoku X - Free Diagonal Sudoku Puzzle Game Online

Sudoku X adds an exciting twist to the classic puzzle: both main diagonals must also contain the numbers 1 through 9 without repetition. This extra constraint means every row, column, 3x3 box, and diagonal must be unique, creating a more challenging and rewarding puzzle experience. Unlimited puzzles, three difficulty levels, all generated instantly in your browser.

How to Play Sudoku X

Sudoku X follows the same basic rules as classic Sudoku, with one key addition: both main diagonals must also contain the numbers 1 through 9 without repetition. The diagonal cells are visually highlighted on the board to help you keep track of this extra constraint.

Whether you're an experienced Sudoku solver looking for a new challenge or trying diagonal Sudoku for the first time, our three difficulty levels -- Easy, Medium, and Hard -- offer a satisfying experience every time.

Basics

  • 1

    Fill the Grid

    Your goal is to fill every empty cell in the 9x9 grid with numbers from 1 to 9. Diagonal cells are tinted purple.

    1
    5
    2
    9
    4
    7
    6
    3
    8
    6
    5
    1
    7
    4
    8
    3
    9
    1
    7
    4
    2
  • 2

    Follow the Standard Rules

    Each row, column, and 3x3 box must contain the numbers 1-9 exactly once. No repeats allowed!

    Each Row

    5
    3
    4
    6
    7
    8
    9
    1
    2

    Each Column

    5
    6
    1
    8
    4
    7
    9
    2
    3

    Each 3x3 Box

    5
    3
    4
    6
    7
    2
    1
    9
    8
  • 3

    Check the Diagonals

    Both main diagonals must also contain the numbers 1-9 exactly once. The X pattern is visible on the board.

    1
    9
    2
    8
    3
    7
    4
    6
    5
    6
    4
    7
    3
    8
    2
    9
    1

    Both diagonals must contain 1-9 uniquely. The center cell belongs to both.

  • 4

    Complete the Puzzle

    Fill in all the cells correctly to complete the puzzle. Every row, column, 3x3 box, and both diagonals must be valid. Try to beat your best time!

    1
    8
    4
    9
    6
    3
    7
    2
    5
    5
    2
    6
    4
    9
    7
    3
    8
    1
    9
    3
    7
    1
    2
    8
    6
    5
    4
    2
    1
    8
    3
    5
    6
    4
    9
    7
    3
    6
    9
    7
    5
    4
    8
    1
    2
    7
    4
    5
    8
    1
    2
    9
    6
    3
    4
    5
    1
    6
    8
    9
    2
    3
    7
    6
    9
    3
    2
    7
    1
    5
    4
    8
    8
    7
    2
    5
    3
    4
    1
    6
    9

Tips & Strategies

Start with the Diagonals

The diagonal constraint adds extra information. Cells where the two diagonals intersect (the center cell) are part of a row, column, box, and both diagonals -- giving you five constraints to work with.

Use Diagonal Elimination

When you place a number on a diagonal, eliminate it from all other cells on that same diagonal. This extra elimination step often reveals placements you would miss in standard Sudoku.

Use Pencil Marks

For harder puzzles, use notes mode to mark all possible numbers in each cell. Remember to account for diagonal constraints when eliminating candidates on highlighted cells.

Corner and Center Cells

Corner cells belong to a diagonal, a row, a column, and a box. The center cell belongs to both diagonals. These highly constrained cells are often the easiest to solve first.

Controls

Mouse / Trackpad

Click on any empty cell to select it, then click a number (1-9) to fill it in. Click the pencil icon to toggle notes mode, and use Clear to remove a number.

Touch Screen

Tap any empty cell to select it, then tap a number to place it. Tap the pencil icon to enable notes mode for marking possible candidates in each cell.

Frequently Asked Questions

What is Sudoku X?

Sudoku X (also known as Diagonal Sudoku) is a variant of the classic 9x9 Sudoku puzzle with an additional constraint: both main diagonals must also contain the numbers 1 through 9 without repetition. This means you need to satisfy row, column, 3x3 box, and diagonal rules simultaneously.

How is Sudoku X different from Classic Sudoku?

Classic Sudoku requires each row, column, and 3x3 box to contain 1-9 uniquely. Sudoku X adds two extra constraints: the main diagonal (top-left to bottom-right) and the anti-diagonal (top-right to bottom-left) must also each contain 1-9 without repetition. This makes the puzzle more constrained but also provides additional logical deductions.

What are the diagonal rules in Sudoku X?

In Sudoku X, the two main diagonals of the 9x9 grid act as additional groups:

Main Diagonal

Top-left to bottom-right

Must contain 1-9 uniquely

Anti-Diagonal

Top-right to bottom-left

Must contain 1-9 uniquely

What do the difficulty levels mean?

Difficulty is determined by the number of pre-filled clues given at the start:

🌱

Easy

43 clues given

Great for learning diagonals

🌿

Medium

35 clues given

A balanced challenge

🌳

Hard

29 clues given

For experienced solvers

Is my progress saved?

Yes, your current game progress is automatically saved in your browser's local storage. If you close the page and come back later, you can continue right where you left off. Your best times for each difficulty level are also saved.

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