Gameplay in Kingdom Hearts III is similar to its predecessors, with hack and slash combat, along the lines of the system seen in Kingdom Hearts II, with an evolution similar to what was seen from Kingdom Hearts to Kingdom Hearts II, and closely tied to the gameplay in Kingdom Hearts 3D: Dream Drop Distance.
Sora returns as the main playable character, once again joined in the party by Donald Duck and Goofy, with the ability to have two additional characters join the party for a total of five-player parties. This is an increase from previous main entries in the series, where players were limited to two additional party members at any given time along with Sora. Sora can perform magic, with a new, powerful tier of each spell available to him, similar to what Aqua has available to her in Kingdom Hearts 0.2: Birth by Sleep – A Fragmentary Passage. Magic performs differently in underwater sections of the game, with Kingdom Hearts III introducing a new spell, Water. Team-up attacks are also featured, which combine Sora and various party members into one attack. Players can also equip various abilities for Sora and his party members. Character summons make a return from previous entries, this time known as "Links", where an additional character joins the battle to assist the player with specialized attacks.
Sora faces Heartless, Nobodies, and Unversed in the game, with new variants of these enemy types appearing. In addition to these traditional "smaller" enemies, the player faces giant bosses, some being weak to summons in the form of theme park rides, known as "Attraction Flow". These attacks are inspired by the Disney Parks attractions Mad Tea Party, Big Thunder Mountain Railroad, Buzz Lightyear Astro Blasters, and Grizzly River Run, and generic pirate ship and carousel attractions. Regarding traversal, the new mechanic, called "Athletic Flow" in Japanese, was refined from the Flowmotion mechanic from Dream Drop Distance. "Athletic Flow" allows the player to utilize the environment to access previously unreachable areas.
Situation Commands also return from previous entries, which can trigger the Keyblade transformations. The Keyblade transformations active the various "Formchanges" – Strike (red), Guardian (yellow), Element (blue), and Blitz (green) – and change Sora's outfit, similarly to the "Drive Form" mechanic in Kingdom Hearts II. Various Keyblades are linked to each form; for example, the Toy Box Keyblade activates Strike Form for Sora, with the Keyblade's transformations changing into a hammer and then into a drill. There is also "Second Form", which changes Sora's battle skills rather than transform the Keyblade. Unlike previous games in the series, players are able to quickly switch between different Keyblades seamlessly during gameplay rather than from the game's equipment menu, and each Keyblade can be leveled up to increase their abilities. Shotlocks also return, which use a focus meter that can target and lock on to individual enemies with the attack depending on which Keyblade is equipped.
Each world offers a specific gameplay element, such as first-person shooter "Gigas" mechs in the Toy Box, downhill sledding in Arendelle, and naval, underwater, and aerial combat in the Caribbean which also features numerous smaller islands at sea which can be explored. The Gummi Ship also returns as a means of travel between the different worlds of the game. The mechanic has been split into two phases: exploration, which has been likened to being more open-world without a fixed travel route as in previous games; and combat, which has increased its scale from previous games, with more enemies present. Gummi Ship customization also returns, with more options available than in previous Kingdom Hearts games.
Various mini-games playable throughout Kingdom Hearts III, including a cooking-themed game featuring Remy from Ratatouille, and a rhythm game in the Kingdom of Corona. Additionally, the "Classic Kingdom" features over 20 games presented in the style of 1980s LCD games such as Game & Watch, while the Hundred Acre Wood world features puzzle games. Kingdom Hearts III also has a "Memory Archive" feature, which has several short films explaining the basic story elements of the series from the previous games.