Play X-Sudoku Online
X-sudoku is an ordinary 9x9 sudoku with a single extra rule: both diagonals must also contain the digits 1 to 9 exactly once. That one rule changes the whole game — it hands you two additional units to eliminate in. At Sudoku Sagax there are 800 levels, ad-free and with no sign-up required.
What is an X-sudoku?
At first glance an X-sudoku looks like any other sudoku: a 9x9 grid, some digits given, the rest to be found. The difference is the two highlighted corner-to-corner diagonals, which together form an X — hence the name. Each of them must also hold every digit from 1 to 9 exactly once.
You will also see it called diagonal sudoku or sudoku X. They all mean the same puzzle.
The extra rule does not necessarily make the puzzle harder, just different. Because two further units exist, there is more to eliminate with — which is why X-sudokus often work with fewer given digits than classic ones. Overlook the diagonals and you stall quickly; use them and you move faster than in a plain sudoku.
The rules, explained simply
The sudoku rules, plus one:
- Rows and columns. Every row and every column contains each digit from 1 to 9 exactly once.
- Blocks. Each of the nine 3x3 blocks contains every digit exactly once as well.
- And the diagonals. In addition, each of the two main diagonals contains the digits 1 to 9 exactly once. They are highlighted on the board.
Watch the centre square: it sits on both diagonals and therefore belongs to four units at once — row, column, block and two diagonals. No square in the puzzle is more constrained.
Techniques for X-sudoku
Everything you know from sudoku still applies. These moves come on top:
- Fill the diagonals first. Treat each diagonal like a row. Three or four digits are often already placed there, and because a diagonal cuts through several blocks, every new digit tells you something about several of them at once.
- Exploit the centre. The middle square belongs to both diagonals. Making progress there moves the whole board, so it is worth working towards deliberately.
- Eliminate diagonally. Before placing a digit, check not only the row, column and block but the diagonal too. Forgetting that fourth check is the most common slip for newcomers.
- Watch the crossings. Where a diagonal passes through a block it occupies three of its squares. If a digit is missing from that block but already appears on the diagonal, it cannot go in any of those three.
X-sudoku at Sudoku Sagax
You will find X-sudoku from the home screen under "All game types". There are 800 levels of its own, a separate leaderboard and its own trophy. Every level you solve counts towards your daily streak and the shared points pool.
The diagonals stay marked on the board, so you never have to keep them in your head. Each puzzle is generated from its level number and has exactly one solution — the same level looks identical for everyone, so best times are comparable.
Notes, error highlighting, hints and undo work as they do in classic sudoku. Everything runs in the browser, ad-free, with no trackers and locally served fonts.
Frequently asked questions
What is an X-sudoku?
An X-sudoku is a 9x9 sudoku with one extra rule: besides rows, columns and 3x3 blocks, both main diagonals must contain each digit from 1 to 9 exactly once. The two diagonals form an X, which gives the puzzle its name. It is also known as diagonal sudoku.
Is X-sudoku harder than normal sudoku?
Not necessarily. The diagonals are two additional units to eliminate in, which helps you solve. That is why X-sudokus often need fewer given digits. It only gets hard if you forget to check the diagonals when placing a digit.
Why is the centre square special?
It lies on both diagonals, so it belongs to a row, a column, a block and two diagonals at the same time. No other square is constrained as tightly, and no digit placed there is without consequences for the rest of the board.
How do you start an X-sudoku?
Start with the diagonals. Treat them as extra rows and see which digits are already there. Because a diagonal runs through several blocks, every digit you add immediately affects several parts of the board.