Skull King is a trick-taking game similar to 'Oh Hell!', 'Wizard', 'Eucher' and 'Spades', with players needing to state how many tricks they think they'll win each round. A unique element of this game is that bidding takes place simultaneously, leading to rounds that are sometimes over, or under-bid. As players must win the exact number of tricks that they bid to earn points, winning too many tricks is just as bad as winning too few and this leads to intense competition where losing a trick can be as exciting as winning one.

Skull King uses a 66-card deck that consists of five Escape cards, four suits numbered 1-13, five Pirate cards, 1 Tigress card, and 1 Skull King card. The game lasts ten rounds, and in each round, each player is dealt as many cards as the number of the round. All players simultaneously bid on the number of tricks they think they'll take by holding out a fist and on the count of three revealing a certain number of fingers (or possibly a closed fist for a bid of zero tricks).

Standard rules apply for the playing of cards, with one player leading off a card and other players following suit, if possible, and playing something else if not; however, a player may always choose to play one of the special, unnumbered cards — and the power of those cards might let you win a trick that otherwise would have gotten away. In more detail, the black Jolly Roger suit trumps the other three suits, a Mermaid trumps all numbered cards, a Pirate trumps the Mermaid, and the Skull King trumps everything — except if he appears in the same trick with a Mermaid, in which case she captures him and wins instead. An Escape card loses to any other card, and the Tigress card serves as either a pirate or escape card as desired by the player. Whoever wins a trick leads in the next trick.

If a player makes his bid exactly, he scores 20 points per trick; if he collects more or fewer tricks, he loses 10 points per trick he's off. If a player makes a bid of zero tricks, he wins points equal to ten times the current round number — but if he takes even a single trick, he loses this many points instead. If a player catches pirates with the Skull King, or the King with a mermaid, he scores bonus points. Whoever has the most points after ten rounds wins.

Additional cards were added in the 2018 to the Grandpa Beck's Games Skull King: Legendary Expansion edition of the game. The artwork was also given a complete refresh. This new version of the game replaces the previous version which will no longer be sold.

Summary of the new cards and rules in Skull King: Legendary Expansion, their inclusion is optional but they add a great deal of additional strategy and intrigue:

The Mermaids (2) beat any numbered card, but lose to any Pirate, with the exception of the Skull King, which can only be captured by a Mermaid.

The Loot Cards (2) function as Escape cards with a special twist: The player who captures a Loot card enters into an alliance with the player who played it down. If both get their bid correct for the round they are each awarded 20 bonus points.

The Kraken destroys any trick in which it is played, with no player winning the trick, regardless of the cards played in that trick.

New abilities have been granted to each of the 5 standard Pirate cards, granting them powers in line with those of the Tigress and the Skull King. Each Pirate's special ability is unlocked by winning a trick in which it is played and must be used immediately thereafter.

Rosie D'Laney: Choose any player to lead out the next trick, including yourself.
Bahij the Bandit: Draw 2 cards, then discard any 2 cards.
Rascal of Roatan: You may choose to wager 10 or 20 points. Win the points if you get your bid right, lose them if you get your bid wrong.
Juanita Jade: Look through all the cards not dealt that round.
Harry the Giant: You may choose to change your bid by +/- 1.

The expansion rules/score sheets also allow for flexible rounds. Players may choose to play the standard 10 rounds with an additional card dealt each round, or any other combination of cards/rounds. For example players can now choose to play 5 rounds with 10 cards dealt each round, or 8 rounds with 2,2,4,4,6,6,8 and 8 cards in a round, or 10 rounds with 5 cards dealt each round, whatever you'd prefer.

Reference cards, scoring summary cards, and bid reminder cards have also been added to the new Legendary Expansion version of the game to facilitate easier play for new players.