MINIX is a Unix-like operating system based on a microkernel architecture, first released in 1987 and written by American-Dutch computer scientist Andrew S. Tanenbaum. It was designed as a clone of the Unix operating system and one that could run on affordable, Intel 8086-based home computers; MINIX was targeted for use in classrooms by computer science students at universities.

Its name comes from mini-Unix. MINIX was initially proprietary source-available, but was relicensed under the BSD 3-Clause to become free and open-source in 2000. MINIX was ported to various additional platforms in the 1990s, and version 2.0 was released in 1997 and was the first to be POSIX compliant. Starting with MINIX 3, released in 2005, the primary aim of development shifted from education to the creation of a highly reliable and self-healing microkernel OS.

Andrew S. Tanenbaum created MINIX at Vrije Universiteit in Amsterdam to exemplify the principles conveyed in his textbook, Operating Systems: Design and Implementation (1987). (Despite sharing a name, it has no relation to the older MINIX from Digital Systems House, Inc. based on AT&T Unix code.)

An abridged 12,010 lines of the C source code of the kernel, memory manager, and file system of MINIX 1.0 are printed in the book. Prentice-Hall also released MINIX source code and executable binaries on floppy disk with a reference manual. MINIX 1 was system-call compatible with Seventh Edition Unix.

Tanenbaum originally developed MINIX for compatibility with the IBM PC and IBM PC/AT microcomputers available at the time.

There is a version of MINIX floating around that supports the Peripheral Technology PT68K-2 and PT68K-4 computers. The PT68K-2 and the PT68K-4 are both 68000 based computers with a standard 8 bit IBM PC ISA bus that has 6 connectors on the main board. It was ported to the PT68K machines by Gary Mills and Sidney Thompson. The port was adapted from the Atari port since it too is a 68000 based machine. The PT68K version has added support for the PT XT-IDE card from Peripheral Technology. It currently only supports the MDA display adapter (no CGA, EGA or VGA). For this reason, it has not yet been ported to the PT68K-5 (aka CDS68020). There is an SD Card image available from https://github.com/mevenson/minix-for-the-PT68K-2-4.

MINIX 1.5, released in 1991, included support for MicroChannel IBM PS/2 systems and was also ported to the 68000 and SPARC architectures, supporting the Atari ST, Amiga, Macintosh, and Sun SPARCstation computer platforms. There were also unofficial ports to Intel 386 PC compatibles (in 32-bit protected mode), National Semiconductor NS32532, ARM and Inmos transputer processors. Meiko Scientific used an early version of MINIX as the basis for the MeikOS operating system for its transputer-based Computing Surface parallel computers.

Demand for the 68k-architectures waned, however, and MINIX 2.0, released in 1997, was only available for the x86 and Solaris-hosted SPARC architectures. It was the subject of the second edition of Tanenbaum's textbook, cowritten with Albert Woodhull and was distributed on a CD-ROM included with the book. MINIX 2.0 added POSIX.1 compliance, support for 386 and later processors in 32-bit mode and replaced the Amoeba network protocols included in MINIX 1.5 with a TCP/IP stack. A version of MINIX running as a user process under SunOS and Solaris was also available, a simulator named SMX (operating system) or just SMX for short.