| SPARC64/OS | HAL Computer Systems | — | — |
| SparkyLinux | — | — | GNU General Public License |
| SPBLinux | — | — | GNU General Public License |
| Special Real Time Operating System | IBM | — | — |
| SPIN | University of Washington | — | — |
| SpiralLinux | — | — | — |
| SPIX | Groupe Bull | — | proprietary license |
| Splashtop | — | — | proprietary license |
| Splashtop OS | Splashtop Inc. | — | — |
| Spring | Sun Microsystems | — | — |
| Sprite | University of California, Berkeley | — | MIT License |
| Stali | Anselm R. Garbe | — | — |
| StartCom Linux | — | — | — |
| StartOS | — | — | — |
| StatiXOS | — | — | — |
| SteamOS | Valve Corporation | — | — |
| Stratus VOS | Stratus Technologies | — | — |
| Subgraph OS | — | — | GNU General Public License |
| Suicide Linux | — | 2009 | proprietary license |
| SunOS | Sun Microsystems | — | proprietary license |
| Sunwah Linux | — | — | — |
| SUPER-UX | NEC | — | — |
| SuperGamer | — | — | — |
| SuperX | — | — | — |
| SuSE Linux | SUSE, Novell | — | GNU General Public License |
| SUSE Linux Enterprise Desktop | SUSE | — | GNU Lesser General Public License |
| SUSE Linux Enterprise Server | SUSE | — | GNU General Public License |
| Svarog386 | — | — | — |
| Sx-window | Sharp Corporation, Hudson Soft | 1989 | — |
| Syllable Desktop | — | — | GNU General Public License |
| Symbian | Symbian Foundation, Nokia, Symbian Software | — | proprietary license |
| SymbOS | — | — | — |
| Symobi | Miray Software | — | — |
| Symphony OS | — | — | GNU General Public License |
| SymplyOS | — | — | — |
| System 1 | Apple Inc. | 1984 | proprietary license |
| System 2 | Apple Inc. | — | — |
| System 3 | Apple Inc. | — | — |
| System 4 | Apple Inc. | — | — |
| System 6 | Apple Inc. | — | proprietary license |
| System 7 | Apple Inc. | — | proprietary license |
| System Software 5 | — | — | — |
| SystemRescue | — | — | GNU General Public License |
| T-Kernel | — | — | — |
| T2 SDE | René Rebe | — | GNU General Public License |
| Tails | The Tor Project, Inc | 2009 | GNU General Public License v3.0 or later |
| Tanglu | — | — | — |
| TeaLinuxOS | — | — | — |