TRSDOS (which stands for the Tandy Radio Shack Disk Operating System) is the operating system for the Tandy TRS-80 line of eight-bit Zilog Z80 microcomputers that were sold through Radio Shack from 1977 through 1991. Tandy's manuals recommended that it be pronounced triss-doss. TRSDOS should not be confused with Tandy DOS, a version of MS-DOS licensed from Microsoft for Tandy's x86 line of personal computers (PCs).

With the original TRS-80 Model I of 1977, TRSDOS was primarily a way of extending the MBASIC (BASIC in ROM) with additional I/O (input/output) commands that worked with disk files rather than the cassette tapes that were used by non-disk Model I systems. Later disk-equipped Model III computers used a completely different version of TRSDOS by Radio Shack which culminated in 1981 with TRSDOS Version 1.3. From 1983 disk-equipped TRS-80 Model 4 computers used TRSDOS Version 6, which was a development of Model III LDOS by Logical Systems, Inc. This last was updated in 1987 and released as LS-DOS 6.3.

Completely unrelated was a version of TRSDOS by Radio Shack for its TRS-80 Model II professional computer from 1979, also based on the Z80 and equipped with 8-inch disk drives. The later machines in this line, the Models 12, 16 and 6000, used the Z80 as an alternate CPU to its main Motorola 68000 chip and could run this version of TRSDOS for backwards compatibility with older Z80 applications software.