| MacOS Sierra | Apple Inc. | 2016 | Apple Public Source License |
| MacOS Sonoma | Apple Inc. | — | — |
| MacOS Tahoe | — | — | — |
| MacOS Ventura | Apple Inc. | 2022 | — |
| Macpup | — | — | GNU General Public License |
| Madbox Linux | — | — | — |
| Maemo | Nokia | — | — |
| Maemo Leste | — | — | — |
| Mageia | — | — | GNU General Public License |
| Magic Cap | General Magic | 1994 | — |
| Magic Linux | — | — | — |
| Magic UI | Honor | — | — |
| MagicOS | Honor | 2016 | proprietary license |
| MagicOS 10 | Honor | — | — |
| MagicOS 9 | Honor | — | — |
| MagicSix | — | — | — |
| MagpieOS | — | — | — |
| MAid Linux | — | — | — |
| MakuluLinux | — | — | — |
| Mandriva Linux | Mandriva | — | GNU General Public License |
| Mangaka Linux | — | — | — |
| Manjaro | Philip Müller | 2011 | free software |
| MaRTE OS | University of Cantabria | — | GNU General Public License, version 2.0 |
| MASTAR | STMicroelectronics | — | — |
| Mastercam | — | — | — |
| Maui Linux | — | — | — |
| Mayix XliveCD | — | — | — |
| MCC Interim Linux | — | — | — |
| McKernel | RIKEN | — | GNU General Public License, version 2.0 |
| MCNLive | — | — | — |
| Mcube Kernel | — | — | 2-clause BSD License |
| MDOS | — | — | — |
| MEDINA | T-Systems International GmbH | — | — |
| MeeGo | Linux Foundation, Intel, Novell, Nokia, AMD | — | GNU General Public License, version 2.0 or later |
| MeNTOPPIX | — | — | — |
| MenuetOS | — | — | proprietary license, GNU General Public License, version 2.0 |
| MEPIS | Warren Woodford | — | GNU General Public License |
| Meta Horizon OS | Meta Platforms | — | — |
| METALIVE | Dream cast, PRIME HOLDINGS | — | — |
| Mezzano | — | — | MIT License |
| Michigan Terminal System | University of Michigan | 1967 | — |
| Micro T-Kernel | T-Engine Forum | — | — |
| MicroBSD | — | — | BSD licenses |
| MicroC/OS-III | — | — | — |
| MicroDOS | — | — | — |
| MicroOS | — | — | — |
| Microsoft Forefront | — | — | — |
| Microsoft Windows | Microsoft | 1985 | proprietary license |