The primary mechanic of Ruthless is deck building. Every turn a player must play a card or set of cards from their hand after which they may buy new cards to their deck. The deck consists of pirate cards, which allow to attack opponent, draw more cards and perform many more profitable actions, and treasure cards which allow to buy more and better pirates. Treasures can be discarded immediately to acquire gold when needed or added to the deck to benefit repeatedly from their bonus. New crew members are laid out in front of the players and their effects are used at the moment of acquisition.
At the end of each round the players will form poker combinations with the cards in front of them and compare the strength of their crew in play. The player with the strongest crew wins the battle and gains most points.
After five or six rounds (depending on player count) the game is over and the player with most points wins.