June 13, 2026

How to Play Sudoku Online — Free, No Login Required

Sudoku is the perfect commute game: focused, finite, satisfying. ShadowBot's Sudoku runs free in any browser, no signup, with three difficulty modes and pencil-mark support. This guide walks through the rules and the basic solving strategies that get you from Easy to Expert.

The Rules

9×9 grid divided into nine 3×3 boxes. Some squares are pre-filled. Fill the rest so that every row, every column, and every 3×3 box contains the digits 1 through 9 exactly once. No guessing required — every valid Sudoku has exactly one solution.

Strategy 1 — Scan for Singles

For each digit 1-9, check whether any row, column, or box has only one empty cell that can hold it. Fill those first. On Easy boards, this alone solves the whole puzzle.

Strategy 2 — Pencil Marks

For every empty cell, list the digits it could possibly contain. When a cell's possibility list collapses to one digit, that's the answer. The ShadowBot board supports pencil-mark mode with a tap.

Strategy 3 — Hidden Pairs

If two cells in a row, column, or box are the only ones that can hold two specific digits, those digits must go in those two cells (in some order). This eliminates them as possibilities everywhere else.

Play Now

ShadowBot's Sudoku has three difficulty modes — Easy, Medium, Hard — each with a fresh puzzle every load. Mistake checker is optional. Mobile-friendly.

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All games on ShadowBot are free, browser-based, and need no signup.

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