Free online anti-knight sudoku — hard difficulty
Anti-Knight Sudoku follows all standard Sudoku rules, plus one extra constraint: no two identical digits can be a chess knight's move apart (an L-shape: 2 squares in one direction + 1 square perpendicular).
In the diagram above, if the center cell contains a 5, then none of the red cells can also contain a 5.
Hard Anti-Knight Sudoku puzzles are where things get serious. With only about 28 given digits, these puzzles will test your logical reasoning and force you to think several steps ahead. The knight constraint becomes both a challenge and a lifeline — you'll need it to solve cells that would otherwise be impossible to determine.
At this level, basic scanning and elimination won't be enough. You'll need techniques like naked pairs, hidden pairs, X-wings, and Y-wings — all modified to account for the knight constraint. One powerful technique unique to Anti-Knight Sudoku: when a digit has only 2-3 possible positions in a region, check if the knight-attack pattern eliminates some of them.
Hard Anti-Knight Sudoku puzzles can take 15-30 minutes to solve. Don't rush. Use the notes feature extensively to track candidates. When you get stuck, don't guess — instead, re-scan systematically and look for any knight conflicts you might have missed. If this feels overwhelming, try medium Anti-Knight Sudoku first.
Once you can consistently solve hard puzzles without hints, it's time to try expert Anti-Knight Sudoku. These are the toughest puzzles on our site and will push your logical reasoning to its limits. Also check out our complete Anti-Knight Sudoku rules guide for technique refreshers.