| R9 | — | — | MIT License |
| RadiOS | — | — | — |
| Rainbow-OS | Ulm University | — | — |
| RAMON | — | — | — |
| RancherOS | Rancher Labs | — | Apache Software License 2.0 |
| Raspberry Pi OS | Raspberry Pi Holdings | 2012 | GNU General Public License |
| Raspbian | — | — | — |
| RavynOS | — | — | — |
| RDOS | Data General | — | — |
| ReactOS | Amine Khaldi, Timo Kreuzer, Eric Kohl, Hermès Bélusca-Maïto, Cameron Gutman | — | GNU Lesser General Public License, GNU General Public License |
| RebornOS | — | — | — |
| Recalbox | — | — | PRS Custom |
| Red Flag Linux | — | — | — |
| Red Hat Enterprise Linux | Red Hat | 2002 | — |
| Red Hat Linux | Red Hat | 1994 | GNU General Public License |
| RED OS | — | — | commercial software |
| Red Star OS | Korea Computer Center | — | — |
| Redcore Linux | — | — | — |
| RedHawk Linux | — | — | — |
| Redox | Jeremy Soller | — | MIT License |
| RedSleeve | — | — | GNU General Public License |
| Refracta | — | — | GNU General Public License, version 2.0 |
| Regata OS | — | — | — |
| Remix OS | — | — | — |
| REMnux | Lenny Zeltser | — | — |
| Replicant | — | — | Apache Software License 2.0, GNU General Public License, version 2.0 |
| Research Unix | Bell Labs, Ken Thompson, Dennis M. Ritchie, Brian Kernighan | — | — |
| Resurrection Remix OS | — | 2012 | — |
| RetroPie | — | — | GNU General Public License |
| REX OS | Qualcomm | — | — |
| Rhapsody | Apple Inc. | — | — |
| Rhino Linux | — | — | — |
| RIOT | Martin Lenders, Benjamin Valentin, Alexandre Abadie | — | GNU Lesser General Public License, version 2.1 |
| RISC iX | Acorn Computers | — | — |
| RISC OS | Acorn Computers, RISCOS Ltd, RISC OS Open, Castle Technology | — | freeware |
| RMX | TenAsys, Intel | — | — |
| Robigalia | — | — | — |
| ROCK Linux | — | — | GNU General Public License |
| Rockbox | — | 2002 | GNU General Public License, version 2.0 |
| Rocks Cluster Distribution | San Diego, University of California, San Diego Supercomputer Center | — | — |
| Rockstor | — | — | — |
| Rocky Linux | Rocky Enterprise Software Foundation | — | 3-clause BSD License |
| Rodos | German Aerospace Center | — | BSD licenses |
| Roku OS | Roku, Inc. | 2004 | — |
| ROM-DOS | Datalight | — | — |
| ROSA Linux | LLC "STC IT ROSA" | 2011 | GNU General Public License, version 2.0 |
| RouterOS | MikroTik | — | — |
| RPath | — | — | — |
| RSTS/E | Digital Equipment Corporation | — | — |
| RSX-11 | Digital Equipment Corporation | — | — |
| RT-11 | Digital Equipment Corporation | — | proprietary license |
| RT-Thread | — | — | Apache Software License 2.0 |
| RTEMS | Sebastian Huber, Joel Sherrill | — | GNU General Public License |
| RTMOS | General Electric | — | — |
| RTX | IntervalZero | — | — |
| RTXC Quadros | — | — | — |
| RubyFPV | — | — | — |
| RunnerOn | — | — | — |
| Runtu | — | — | — |
| Russian Fedora Remix | — | — | — |