Free Classic and Killer Sudoku: daily puzzles, live tournaments, 1v1 duels and a coach for every technique — running entirely on the Internet Computer blockchain.
What is Sudoku?
Sudoku is one of the most beloved logic puzzles in the world. The goal is beautifully simple: fill a 9×9 grid so that every row, every column, and every 3×3 box contains the digits 1 through 9 exactly once. No math required — just logic, pattern recognition, and patience.
First published in the United States in 1979 and popularized in Japan in the 1980s under the name "Sudoku" (meaning "single number"), it has since become a global phenomenon enjoyed by millions. It sharpens critical thinking, improves concentration, and provides a deeply satisfying sense of accomplishment with every completed puzzle.
What Makes Us Different?
Sudoku Challenge isn't just another Sudoku app. It has two full game modes — Classic Sudoku and Killer Sudoku — across five difficulty levels, daily puzzles with streaks and leaderboards, real-time tournaments, head-to-head duels and a skill-based rating. It runs on decentralized blockchain technology: you sign in without an email or password, and your game data lives in on-chain smart contracts rather than a private company database.
Classic & Killer Sudoku
Two full game modes. Play Classic Sudoku across five difficulty tiers, or switch to Killer Sudoku — cages with a target sum and no repeated digit — with its own five tiers, a live cage-combinations helper, and a step-by-step teacher for every deduction.
Real-Time Tournaments
Create or join a tournament lobby — Easy, Medium or Hard, from 15 minutes to an hour — and race other players through timed puzzles. Guests can join too; signed-in players earn rating.
1v1 Duels
Challenge a specific player by name or join the Pool Match queue to find an opponent. Direct challenges are available to rated players within 200 rating points of each other. Same puzzle, same clock — pure skill decides the winner.
Daily Challenges & the Daily Triple
Every day brings Easy, Medium and Hard puzzles in both Classic and Killer, plus the Classic Daily Challenge — one Hard puzzle a day with a leaderboard. The Daily Triple adds three elite puzzles — Classic, Symmetric and Shape, each Extreme or Diabolical — with one combined leaderboard. Keep a streak going and earn badges at 7, 30, 100 and 365 days.
Skill-Based Ratings & Tiers
Earn a competitive rating and progress through tier ranks from Beginner to Super Grandmaster. Your rating reflects your true Sudoku skill level.
Built-In Solver & Importer
Enter any Sudoku puzzle and get an instant solution, with a check that it has exactly one. Import puzzles from your camera, an image, or plain text, then play them with the Next Step coach.
Game Replay & Sharing
Watch any game again move by move, pencil marks included, and share the link with friends. Saved puzzles keep your digits, pencil marks, colours and time, so you can carry on later.
Live Spectator Mode
Non-participants can watch any player's progress in real time during a live tournament. Open the Live Tournament, view the leaderboard, and click the Watch button next to any player's name. Participants cannot spectate during their own tournament to ensure fair play.
Puzzle Stats
Every puzzle shows how many players have solved it and the fastest time — before you start, so you know what you are up against, and again when you finish.
Fair Play
Competitive replays are checked automatically for solves copied from a solver, the solver refuses any puzzle that is live in a competition, and competitive boards blur when you leave the tab. See the Fair Play policy.
Training Mode & Next Step
Practice Classic and Killer Sudoku across five difficulty levels with a built-in coach. The Next Step button reads your board — including your own pencil marks — and walks you through the next deduction, with highlights and arrows showing the logic: all 28 Classic techniques from Naked Singles to ALS-XZ, plus Killer cage strategies. The Learning Hub has a worked lesson for every Classic technique and the core Killer strategies.
Built on Blockchain
Sudoku Challenge runs entirely on the Internet Computer Protocol (ICP) — a next-generation blockchain that operates at web speed. This isn't a marketing gimmick; it fundamentally changes what the platform can promise you:
Privacy by design
No personal data collected
Decentralized hosting
Runs on blockchain nodes worldwide
No email or password
Sign in with Internet Identity — no account details to give
Web-speed performance
Feels like a native app
Puzzle Quality
Every puzzle on Sudoku Challenge has exactly one solution and is graded by the techniques a person needs to solve it — Classic and Killer alike. There are five difficulty levels, each with its own score range, so you always know how hard a puzzle is:
Easy
Naked Singles only — perfect for beginners
Score: 100 – 999
Medium
Adds Hidden Singles and Locked Candidates
Score: 1,000 – 1,999
Hard
Pairs, Triples, Quads and Unique Rectangles
Score: 2,000 – 4,999
Extreme
X-Wings, Skyscrapers, XY-Wings, Coloring and more
Score: 5,000 – 7,999
Diabolical
XY-Chains, finned fish, X-Cycles and ALS — the hardest logic
Score: 8,000 – 9,999
With 146,764 graded Classic puzzles plus a Killer collection in the same five levels, you'll never run out. Each puzzle is scored from 100 to 9,999 by the techniques it requires, so difficulty is consistent across the whole site. Stuck? Training Mode's Next Step walks you through the logic one deduction at a time. In a tournament or duel every player gets the same puzzle, served by the blockchain.
Every puzzle can be solved using logical techniques. Our Learning Hub covers all 28 Classic techniques — plus Killer cage strategies — with visual worked examples, from beginner-friendly Naked Singles to advanced XY-Chains and ALS-XZ.
More Features
Customizable avatars & country flags
Full rating history chart with milestones
Automatic fair-play checks on competitive replays
Responsive design — play on any device
Live spectator mode — watch any player during a tournament in real time
Share any puzzle or replay with a link
Pencil marks & multi-cell selection
Unlimited undo/redo during play
Print puzzles with or without solution
Import puzzles from camera or image
Premium Plan
Sudoku Challenge is completely free to play — and always will be. Free players have full access to every feature, game mode, and competitive event. There are no ads on the site today.
If ads are ever added, a Premium subscription removes them. Premium is purely about comfort — it does not provide any gameplay advantage, extra features, or competitive edge. Everyone competes on equal footing.
Get in Touch
Have a question, suggestion, or found a bug? We'd love to hear from you.