Knack is a third-person brawler featuring a protagonist that was assembled by a doctor from ancient relics. The creature is made up out of a large amount of independent parts tied together. The story describes how a species called Goblins led by Gundahar attack mankind. Knack joins the industrialist Viktor to fend off the Goblins, working together with security robots. The game generally presents a series of arenas linked together where Knack has to fight off opponents to progress. He attacks with his arms, performing combos and he can also jump and dodge. Throughout the game different types of relics allow Knack to take on alternate forms. As Ice Knack he has more protection, but receives damage through melting. Metal Knack is very strong both defensively and offensively, but magnetism opens up a weakness. Wood Knack needs to watch out for fire and Stealth Knack can travel through lasers undetected. Regular relics allow him to grow in size or reverse the process when he is damaged.

By defeating enemies and smashing yellow crystals, sunstones are collected that fills up symbols. Once full, Knack can unleash a powerful move such as shockwave, storm and blast. He can also transform into a much larger and powerful form. The different Knack types are often triggered automatically for a specific section and are often used for environment based puzzles. There are also platform sections and health can be picked from the environment. The thirteen chapters contain secret rooms that provide additional relics or parts used to create gadgets. These allow Knack to slow down time or enable a damage multiplier for instance. The game uses a fixed camera that changes when a new location of a part of a location is accessed.

It is possible to unlock an alternate Vampire Knack where relics need to be absorbed and picked up regularly to stay alive, as if in a time based mode. The mobile game Knack's Quest can be used to unlock special relics in the game.