Sudoku Online

Sudoku Online is a full-featured sudoku game that runs straight in the browser with no sign-up. Classic mode comes in four difficulties, from Easy for newcomers through Medium and Hard up to Expert, and every puzzle has exactly one solution that can be reached by logic alone, so there is never a need to guess.

Beyond the standard rules there are four variants. X-Sudoku adds both main diagonals as extra constraints, Windoku highlights four additional regions, Killer Sudoku replaces some givens with cages that must add up to a target sum, and Kropki puts dots between cells: a white dot means the two digits differ by one, a black dot means one is double the other. A notes mode lets you pencil in candidates. Leaderboards rank players by level, score and wins, profiles keep your history and statistics, and a chat lets players swap strategies.