Kirby Star Allies is a 2.5D platform game played from a side-on perspective. Players control the series' titular protagonist Kirby who can be accompanied by up to three companions.[1] Kirby can throw hearts at enemies to turn them into allies.[1] The game can be played alone with the game's AI controlling companions or cooperatively with other players controlling companions.[2] When Kirby has companions, new special attacks become available which allow Kirby to combine his abilities with those of his allies', a feature not seen since Kirby: Squeak Squad.[3] The elemental fusions last for as long as the power is held or until a new element is introduced over it, while some of the combinations are single-use moves that bear more of a resemblance to the combinations found in Kirby 64: The Crystal Shards. When Kirby has three allies, they can perform "Friend Actions" on specific stages, like "Friend Train" and "Friend Star."[4] Players start with a certain number of lives and can earn more lives by getting extra lives or by collecting 100 stars. Kirby will lose a life if he falls into a bottomless pit or his health bar is depleted. The game ends when the player runs out of lives, although the player can continue the game from the most recent save point by selecting "Continue".

The game also introduces the "Dream Friend" system, which allows the player to call upon characters from past Kirby games as special allies with expanded movesets. Bandana Waddle Dee, King Dedede, and Meta Knight can all be unlocked as Dream Friends through progression in Story Mode. Additional Dream Friends have been added via software updates.