Macintosh File System (MFS) is a volume format (or disk file system) created by Apple Computer for storing files on 400K floppy disks. MFS was introduced with the Macintosh 128K in January 1984.

MFS was notable both for introducing resource forks to allow storage of structured data as well as for storing metadata needed to support the graphical user interface of Mac OS. MFS allows file names to be up to 255 characters in length, although Finder does not allow users to create names longer than 63 characters. MFS is called a flat file system because it does not support folders.

Apple introduced Hierarchical File System as a replacement for MFS in September 1985. In Mac OS 7.6.1, Apple removed support for writing to MFS volumes, and in Mac OS 8 support for MFS volumes was removed altogether.