| Ubuntu Touch | UBports | 2013 | GNU General Public License |
| Ubuntu Unity | — | 2020 | — |
| Ubuntu Web | — | — | — |
| UbuntuDDE Remix | — | — | GNU General Public License, version 3.0 |
| UCS@school | Univention | — | GNU Affero General Public License |
| UCSD p-System | — | — | — |
| UDOS | VEB Robotron | — | — |
| Uenux | — | — | — |
| Ufficio Zero | — | 2020 | GNU General Public License |
| Uhuru Mobile | — | — | — |
| UKnow4Kids | — | — | GNU General Public License, version 2.0 |
| Ultima Linux | — | — | GNU General Public License |
| Ultimate Edition | — | — | — |
| Ultimate Linux | — | — | — |
| Ultramarine Linux | — | — | — |
| Ultrix | Digital Equipment Corporation | — | — |
| Umbrel OS | — | — | PolyForm Noncommercial License |
| Umbrella Linux | — | — | — |
| UNICOS | Cray | — | — |
| Unics | — | — | — |
| UniFLEX | Technical Systems Consultants | — | — |
| Unity Linux | — | — | — |
| Unity Operating System | — | 2020 | — |
| UNIVAC EXEC I | UNIVAC | — | — |
| UNIVAC EXEC II | Computer Sciences Corporation | — | — |
| Univention Corporate Server | — | — | GNU Affero General Public License |
| Universal Time-Sharing System | Scientific Data Systems | — | — |
| University of Michigan Executive System | University of Michigan | — | — |
| UNIX System III | AT&T Corporation | — | — |
| UNIX System Services | IBM | — | — |
| UNIX System V | AT&T | 1983 | — |
| UNIX/32V | Bell Labs | — | 4-clause BSD License |
| Unix/NS | NCR Corporation | — | — |
| UnixWare | Novell, SCO Group, Caldera, Santa Cruz Operation, Univel | — | proprietary license |
| Unslung | — | — | — |
| Unununium | — | — | BSD licenses |
| UP-UX | NEC | — | — |
| Uruk GNU/Linux | — | — | — |
| USIX | — | — | — |
| USU | — | — | GNU General Public License |
| UTS | Amdahl Corporation, UTS Global | 1981 | — |
| Ututo | — | — | GNU General Public License |
| UwUntu | — | — | Creative Commons Attribution 4.0 International |
| UX/RT | — | — | — |
| UXP/DS | Fujitsu | — | — |
| V operating system | David Cheriton | — | — |
| Vanilla OS | — | — | GNU General Public License, version 3.0 |
| VAXELN | Digital Equipment Corporation | — | — |