Following the game's release, five more One Piece games would be produced for WonderSwan consoles, though Become the Pirate King! remains the only one compatible with the original monochromatic system.
The game mainly revolves around path-building, with players laying tiles across various sea maps. The piecemeal paths depicted on these tiles must be connected so players can reach objectives such as islands and enemy crews.
Tiles come in four basic path formations - straights, elbows, T's, and crosses - that can be rotated clockwise or counterclockwise in 90-degree intervals. A tile's "strength" is determined by its texture; dark tiles (carrying "smooth" rivets) can be replaced by new tiles, while light tiles (carrying "dotted" rivets) cannot.
Players are limited to a pool of five tiles at any given time. After a tile has been laid, its successor must be chosen from a randomly shuffled "deal" of five new tiles.
Exploration mainly relies on the Move (移動 Ido?) command, which opens the player's current tile pool and (if applicable) replacement shuffle. One tile must be laid every turn, though it need not be connected or even adjacent to any other tiles.
After laying the tile, the player will be granted a Jolly Roger marker to plot their destination, and a number of movement points equal to the number on the tile; each rivet on a path costs one movement point to cross. Once the destination is set, the player will automatically sail there, ending the current turn.
Note that "leftover" movement points cannot be carried between turns; every new turn will start with zero movement points.
Combat employs the same general tile mechanics as exploration. Players may field anywhere from one to three characters as combatants, and lay up to three tiles per round.
The combat map proper is a 3x3 grid, with the battling crews on opposite ends. The top and bottom rows of this grid are pre-laid with identical paths; to reach the enemy, each side must build a functional path through the middle row. While movement points do not exist in combat, the numbers on the tiles are still vital for determining initiative; whichever side compiles the higher sum lays - and attacks - first.
Once the tiles are laid, the combatants will move according to their lineup positions: the rightmost character will take the rightmost route, and the leftmost character the leftmost route, and the center character the center route. This lineup may be reordered at the start of every round, alongside tile selection.
Each character is assigned a Type (タイプ Taipu?), which governs their attack capabilities:
- Melee (近距離 Kinkyori?) types, such as Zoro, must reach the far row to attack. On reaching the far row, they will attack the character in the closest square; if this square is unoccupied, they will attack the enemy ship instead.
- Ranged (遠距離 Enkyori?) types, such as Usopp, must stop at the middle row to attack. Unlike Melee types, they can only attack the enemy ship.
- Adept (万能 Ban'nō?) types, such as Luffy, may employ both Melee and Ranged attacks.
Combat performance relies on the following attributes:
- Offense (攻撃力 Kōgeki-ryoku?), which determines the "raw" damage dealt by a successful attack.
- Defense (守備力 Shubi-ryoku?), which determines the damage deducted from an enemy attack; if high enough, it can completely nullify an attack.
- Agility (素早さ Subaya-sa?), which determines what order a given side's combatants will attack in.
In addition, every character has several Finisher (必殺 Hissatsu?) attacks tied to unique path formations - Luffy, for instance, will perform a Gomu Gomu no Bazooka if facing a cross between two vertical straights. A successful finisher will always deal a fixed amount of damage to every enemy combatant and ship, but also lock any allies out of attacking until the next round.
Two win conditions exist: either defeat every member on the enemy crew, or sink the enemy ship.
Four different modes may be accessed from the Top Menu, including several multiplayer modes able to support up to four players. Note, however, that none of these modes support play via the WonderSwan's link-cable system; instead, a single WonderSwan console must be passed back and forth between players.
- The Pirate King and the Great Swordsman (海賊王と大剣ごう Kaizoku-Ō to Daikengō?)
- Femme Fatale (ましょうの女 Mashō no on'na?)
- 800 Lies (ウソ Uso 800?)
- Soup (SOUP?)
- Tears (なみだ Namida?)
- The Legend Begins (伝せつははじまった Densetsu wa hajimatta?)
- The Isle of Gold (おうごんのしま Ōgon no shima?)
Each level contains a mixture of racing missions and boss battles, interspersed with scenes and dialogue heavily drawn from the original manga.
Apart from moving, the player may access the following options:
- Tools (道具 Dōgu?): Utilize (or discard) any items currently in possession.
- Crew (仲間 Nakama?): Display (and rearrange) the current crew roster.
- Detail (情報 Jōhō?): Examine the capabilities and morale of current crew members.
The Together (いっしょ Issho?) mode allows players to assemble custom pirate crews and race each other across eight different maps. Its combat and sailing mechanics are mostly identical to those of Solo mode, although players cannot see enemy ships until they occupy adjacent tiles. Combat initiates whenever two ships meet on the same (non-checkpoint) tile, and is limited to one round in all circumstances.
In addition, players may access several special tiles that carry no paths or movement points, instead laying traps or exercising other effects on gameplay:
- Whirlpool (渦 Uzu?) tiles send the first player to cross back to their starting point.
- Rock (岩 Iwa?) tiles capsize the first player to cross, forcing them to skip their next turn.
- King (王 Ō?) tiles summon a Sea King on the first player to cross, forcing them to skip their next turn.
- Cannon (砲 Hō?) tiles allow the player to target and destroy any one tile within a 5x5 area; any player occupying the targeted tile will be capsized and forced to skip their next turn.
- Wind (風 Kaze?) tiles double the movement points on the next two tiles the player lays.
Trap tiles can be laid onto any path tile - regardless of strength - on the adventure map, and are invisible on all turns apart from the laying player's. They can also be laid on the combat map, dealing a flat amount of damage to the first character to cross.
An electronic adaptation of the Carddass Hyper Battle system attached to the official One Piece card game. Supports up to four players.
- The Solo mode depicts the Going Merry as Luffy's ship from the very beginning, rather than an acquisition from Syrup Village.
- On its initial release, the game's instruction booklet was packaged with six exclusive Carddass cards, all based on the game's box-art. One used the box-art directly, while the other five depicted the individual Straw Hat Pirates.