FFU is an system image format developed by Microsoft. FFU files are used to store Windows Phone system images and can be stored on external storage devices, such as SD cards. FFU files were typically used for flashing Lumia smartphones that ran Windows Phone operating system.

FFU format is seldom used as Windows Phone devices are no longer available on the market. Older devices running Windows Phone still support FFU files, which can be used to restore or update the system.