Sudoku X Online
Classic sudoku with one extra rule — both main diagonals must also contain 1–9 exactly once. Pick a tier and start solving.
Sudoku X — also called Diagonal Sudoku — is the 9×9 variant that adds two extra constraints to the standard sudoku rules. In addition to row, column, and 3×3 box uniqueness, each of the two main diagonals (top-left to bottom-right and top-right to bottom-left) must also contain the digits 1–9 exactly once. On the board the diagonals are picked out with a subtle tint so you can see the constraint you're solving. Sudoku X plays smoother than Classic at the same clue count — the extra diagonal eliminations shorten technique chains, so the difficulty ladder shifts down a notch. Easy is a gentle 8–15 minute introduction; Expert pushes 40+ minutes on the hardest grids. Start anywhere and use the diagonals as a third axis of elimination from move one.