Game file used by Fallout mod programs such as Fallout mod manager (FOMM) and Nexus Mod Manager (NMM); contains files that, when installed, change game content for Fallout 3 or Fallout: New Vegas; used for applying custom mods developed by game community members.

The FOMOD format may use .7Z, .ZIP, or .RAR compression. Within the compressed archive, there is an info.xml file that describes the mod and a script that installs the mod. There are also data files that alter the in-game content. FOMOD files can be installed by double-clicking them when FOMM or NMM is installed. FOMM opens and shows the mod in the content window, and you activate it to apply the mod. Mods used by FOMM are generally installed to the Fallout 3\fomm\mods\ folder within the game installation directory.