Biomutant is the first game of the Swedish development studio Experiment 101, which was established in 2015 by former Avalanche Studios employees. On August 19, 2017, an advertisement in the German gaming magazine GamesMarkt revealed the existence of the game, dubbing it as a "post-apocalyptic kung-fu fable", before being fully announced on August 21. The game was playable later that week at the German trade show Gamescom. In an interview which was published the day after the announcement of the game, Experiment 101's head of studio Stefan Ljungqvist said that the game already was content complete. He also mentioned that one of the most difficult things for the development team was to balance the combat system in order to "combine shooting, melee [attacks], and abilities in a quite intuitive way". Unreal Engine 4 is being used for the game's development.
Biomutant is an action role-playing game set in an open world environment and played from a third-person perspective in which the player takes control of a raccoon-like creature in a world filled with mutated animals. The first thing for the player to do is to customise their own character. Gender, length, thickness, fur, fangs, and many other attributes can be tweaked and all have direct impact on the statistics of the playable character during gameplay. A thicker character is heavier, which makes it slower but on the flip-side allows it to deal and resist more damage. The combat system combines melee attacks with long-range shooting. The player can choose between a variety of swords and guns, and combine their power by switching between different weapons during combat. The protagonist is able to dodge to the left, right, and backward, as well as jump. Besides leveling up the playable character, the player can also use level-up points to strengthen their attributes or to unlock new attack combinations. Another way to learn new moves is by interacting with specific characters in each area. Most moves are based on weapons, which can be crafted using the game's weapon crafting system. Each weapon has sockets to put add-ons onto, such as a corkscrew or a small chainsaw. The game features a variety of Affixes, including cryogenic and electrical, which are effective in battles. When a cryogenic add-on is put onto a weapon, enemies will freeze when hit.
Throughout the game, players can also change their abilities, looks by mutations and bio-mechanical body parts such as attaching robotic legs, wings and claws which can be used to get various advantages against certain enemies or at certain locations. In order to reach specific areas, the player must change body parts, equip gear that is suited for that area, or obtain specific vehicles, such as an air balloon or jet skis. This way, the player can overcome obstacles introduced by their surroundings, such as the limited oxygen availability in Death Zone: by wearing a gas mask or an oxygen container, the player is able to venture deeper into the area and explore new places that cannot be reached without any precautions, while an even bigger part of the area can be reached by obtaining a mech. The game's world can be seamlessly explored by the player on foot, by flying or using an air balloon, jet ski and mech. It also features a dynamic weather system and day-night cycle which affects the gameplay and enemy behavior. The story is mission-based and mission lines are character-based. Certain characters will provide the player quest lines that are emanating from their area. More the player interacts with this character, the wider the arc of that specific area becomes.
Biomutant also features a Karma system for non-playable characters and a companion system for recruiting allies for fighting and travelling alongside with the player. Depending on the actions, interactions, and decisions made by the player throughout the whole game, the stances characters take can change, which has an influence on the continuation of the story as dialogues and quest lines will be changed. The story is fully narrated. As playtime increases, the narration level scales down. Players will also be able to manually adjust the narration frequency.
Biomutant has branching storylines where decisions made by the player will decide how the story will continue. The world of Biomutant is struck by a natural disaster as poisonous oil comes up from beneath the surface and pollutes the Tree of Life. The Tree of Life has five roots through which it gives life to the whole world. In order to save the Tree of Life, players need to go to the end of each root, where aside from the oil a creature is destroying the root by gnawing at it. There are also six tribes that are split from their original enclave. Three of them want to heal the Tree of Life, while the three others plan to do things on their own. Each tribe can be influenced through the Karma system. The player can ally with a tribe and eliminate other tribes in order to grow the power of the allied tribe in the world, although the player can also decide on the Tree of Life's fate without having to murder any of the tribes if they want to.