Samurai Sudoku: How to Play the Five-Grid Variant

Samurai sudoku is the most popular sudoku variant after the classic 9×9. It looks intimidating, with five interlocking grids arranged in an X pattern and 369 cells in total, and the first impression is usually that it’s five sudokus stacked together. It isn’t. The four corner grids share boxes with the central grid, which means a placement in any shared cell satisfies constraints in two grids at once. That sharing is what makes the puzzle distinct, and it’s where the strategy diverges from classic sudoku.

The structure

A samurai sudoku consists of five 9×9 grids:

• One central grid.
• Four corner grids, one each at top-left, top-right, bottom-left, and bottom-right.

Each corner grid shares a single 3×3 box with the central grid. The shared box is placed at the corner where the two grids meet: the top-left corner grid shares its bottom-right 3×3 box with the central grid’s top-left 3×3 box, and so on. Four shared boxes in total. Total cells: 81 × 5 minus 9 × 4 (the overlaps) = 369.

The arrangement looks like an X, or a stylised samurai helmet crest, which is where the name comes from. The variant emerged from the broader Japanese tradition of overlapping sudoku puzzles in the late 1990s, and was popularised in the West in the mid-2000s alongside the classic puzzle.

The rules

The rules are exactly the classic sudoku rules, applied independently to each of the five grids. There are no new rules. Specifically:

• Every row of every grid must contain the digits 1–9 exactly once.
• Every column of every grid must contain the digits 1–9 exactly once.
• Every 3×3 box of every grid must contain the digits 1–9 exactly once.

The trick is that a cell in a shared box belongs to both of the grids that overlap there. A digit placed in a shared cell must satisfy the row, column, and box constraints of both grids simultaneously. That’s the entire mechanism of the puzzle.

What overlapping boxes change

In classic sudoku, every cell sits at the intersection of three constraints: a row, a column, and a box. In a samurai sudoku shared cell, a cell sits at the intersection of six constraints: two rows (one from each grid), two columns, and two boxes. (The two boxes are physically the same 3×3 region, but the corner-grid’s row/column structure is different from the central-grid’s.)

Practically, this means the shared boxes are the most constrained regions on the entire puzzle. They are the natural place to start solving.

How to actually solve one

A samurai sudoku looks intimidating because of the size, but the right approach makes it tractable. The structure has clear starting points and a clear solving rhythm.

1. Start with the shared boxes

Each shared 3×3 box belongs to two grids at once. That’s six rows, six columns, and two box constraints intersecting on nine cells. Those cells are heavily constrained, and they almost always yield placements early.

Inside a shared box, you have two perspectives at once. From the corner grid’s perspective, the box is one of nine; the rows and columns of the corner grid run through it. From the central grid’s perspective, the same box is also one of nine, but with completely different rows and columns. A digit that’s blocked by a corner-grid row will appear blocked from one direction; a digit blocked by a central-grid column will appear blocked from the other. Place digits in the shared box only when you can verify both grids would accept them.

2. Solve each grid mostly independently

Once shared boxes have started filling in, each of the five grids becomes a slightly-easier-than-classic sudoku, because nine of its eighty-one cells are already partially constrained by the shared box. Solve each grid using the same techniques as classic sudoku (cross-hatching, hidden singles, locked candidates), described in our techniques guide.

The corner grids tend to fall first, because each is constrained by only one shared box. The central grid is constrained by all four shared boxes simultaneously, which is helpful but harder to track.

3. Bounce between grids

Every time you place a digit in a shared box, it propagates constraints into both adjoining grids. A solver who has been working entirely in the top-left grid can suddenly unlock cells in the central grid because of a single placement in the shared box. The fluent rhythm of samurai sudoku is bouncing between grids whenever a shared placement is made.

4. Track conflicts carefully

The biggest beginner mistake in samurai sudoku is placing a digit that’s valid for one grid but conflicts in the adjoining grid. Always verify shared-cell placements against both grids’ rows and columns. On paper, this means scanning twice as many lines. In an app, the highlighting usually makes conflicts visible immediately, which is one of the reasons digital samurai sudoku is dramatically easier than paper.

Difficulty in samurai sudoku

Samurai sudoku doesn’t have a single difficulty rating; it usually has five, one per grid. A samurai puzzle described as “hard” typically has at least one grid requiring hard-difficulty techniques, but the other four may be medium or easier. Total solve time is roughly two to three times that of a classic sudoku of the hardest sub-grid’s rating, not five times, because the shared cells make each individual grid easier than it would be standalone.

In Sudoku Samurai (the app), every region boss in the campaign is fought as a samurai sudoku, with difficulty calibrated to the region you’re currently in. The structure mirrors how traditional samurai sudoku books rate their puzzles: by the hardest sub-grid, not the average.

Why the variant works

Samurai sudoku is, in our biased opinion, the most elegant of the major sudoku variants, because it adds difficulty without adding rules. Killer sudoku adds arithmetic; jigsaw sudoku replaces the box structure; anti-knight adds a chess-derived constraint. Samurai sudoku adds nothing; it simply glues five copies of classic sudoku together at carefully chosen seams. The mathematics, the deductions, the techniques are all the same. Only the geometry has changed.

That’s probably why it lasted. Variants that add rules eventually feel arbitrary; samurai sudoku feels structural, like a natural extension of the classic puzzle. Once you’ve solved a few, the X shape starts to look as inevitable as the 3×3.

A few practical tips

• Shared boxes first, always. They’re the highest-information regions. Don’t start with a corner grid in isolation.
• Don’t pencil-mark the whole puzzle. 369 cells of pencil marks is unmanageable. Mark a single grid at a time, ideally one near a recently-updated shared box.
• Watch for cross-grid hidden singles. When a shared box fills in, scan the row and column of both grids that pass through it. New placements often reveal hidden singles two grids away.
• If stuck, restart from a shared box. The highest-information regions are also the easiest places to recover from solving fatigue. A fresh look at one of the four shared boxes usually finds the next move.

Samurai sudoku is the variant we built our app’s campaign around: every region boss is a samurai sudoku, scaled to the difficulty of the province you’ve unlocked. If you’re curious how that works, the campaign system is described on our homepage. And if you want to sharpen the underlying solving techniques first, the techniques guide covers everything from singles to X-Wings.