The game is being developed with Unreal Engine 5, whereas Avalanche has historically used its own Apex engine for Just Cause, Mad Max, and Rage 2. Tech Director Fredrik Lönn explained the engine choice with the following statement:
Custom engines were historically the way forward, and that's what my former team at Avalanche did over 18 years ago when development started on Just Cause 1. The landscape has changed since then, and now it's quite the opposite. We decided to go with Unreal Engine 5. Unreal has been proven for high-quality AAA games, both by Epic themselves in the case of Fortnite and by other game studios worldwide shipping amazing games on Unreal. With Epic making improvements toward open-world development, we get full access to the source code, and the integrations with other software and pipelines are excellent.