| RTX | IntervalZero | — | — |
| RTXC Quadros | — | — | — |
| RubyFPV | — | — | — |
| RunnerOn | — | — | — |
| Runtu | — | — | — |
| Russian Fedora Remix | — | — | — |
| S5-DOS/MT | Siemens | — | — |
| S60 | Nokia | — | — |
| Sabayon Linux | — | — | GNU General Public License |
| Sabily | — | — | GNU General Public License |
| SabineOS | — | — | — |
| Sacix | — | — | GNU General Public License |
| Sailfish OS | Jolla | — | — |
| Salix OS | — | — | GNU General Public License, version 3.0, GNU General Public License, version 2.0, Creative Commons Attribution-ShareAlike 3.0 Unported, BSD licenses |
| SambaEdu | — | — | GNU General Public License |
| SAMDOS | Miles Gordon Technology | — | — |
| SAMity Linux | — | — | — |
| Samsung Experience | Samsung Group | — | — |
| Satux | — | — | — |
| SchilliX | — | — | Common Development and Distribution License version 1.0, GNU General Public License |
| Scientific Linux | Fermilab | — | GNU General Public License |
| SCore | RWCP | — | — |
| Scout | University of Arizona | — | — |
| ScreenOS | Juniper Networks | — | — |
| SeaBIOS | — | — | GNU Lesser General Public License, version 3.0 |
| Secureblue | — | — | Apache Software License 2.0 |
| Security Onion | — | — | — |
| Seminarix | — | — | — |
| Semplice Linux | — | — | — |
| SEP OS | Apple Inc. | — | — |
| Serbian GNU/Linux | — | — | — |
| SerenityOS | Andreas Kling | — | 2-clause BSD License |
| Series 30 | — | — | — |
| Series 30+ | — | 2013 | — |
| Series 40 | — | — | — |
| SHARE Operating System | SHARE | — | — |
| SharpOS | — | — | GNU Lesser General Public License, BSD licenses, GNU General Public License, version 2.0 |
| Shinux | — | — | free software license |
| SHR | — | — | GNU General Public License |
| ShredOS | — | — | GNU General Public License, version 2.0 |
| Shrine | — | — | Unlicense |
| SIBO | Psion | — | — |
| Siduction | — | 2011 | GNU General Public License |
| Sigma File Manager | — | — | GNU General Public License v3.0 or later |
| SimpleLinux | — | — | — |
| Sinclair QDOS | Sinclair Research | 1984 | proprietary license |
| Single User Control Program | VEB Robotron | — | — |
| SINIX | Fujitsu Siemens Computers, Siemens Nixdorf Informationssysteme | — | — |