The Operating System (TOS) was the operating system of the Atari ST range of computers. This range included the 520ST, 1040ST and the F, FM and E variations (e.g.1040STE). Later, 32-bit machines (TT, Falcon030) were developed using a new version of TOS, called MultiTOS , which was based on MiNT.

TOS combined Digital Research's GEM GUI running on top of the DOS-like GEMDOS. Features included a flat memory model, MS-DOS-compatible disk format, support for MIDI, and a variant of SCSI called ACSI (in later versions).

There is some argument as to what TOS stands for, the main candidates being "Tramiel Operating System" (named after Jack Tramiel, Atari's head at the time) or simply "The Operating System".