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According to an announcement on Steam Community, Valve has added native support for Xbox controllers on Steam Client Beta, specifically support for Xbox 360 and Xbox One controllers, and generic X-Input gamepads. With this update, players will be able to map features using Valve’s Steam Controller Configurator.
Though Valve has been offering Xbox controller support for Steam, it has been on a game-to-game basis. The newest beta upgrade simplifies the process, offering support for more controllers and more freedom to customize controller setup before gaming.
Valve also added support for third-party PS4 flight sticks and game controllers made by HORI, MadCats and other brands. PS4 controller support has been available since last year.
[Source]: Steam Community: Steam Client Beta Update – Jan 5th.