| TeaM-TL | — | 2004 | — |
| TeleSec Chipcard Operating System | T-Systems International GmbH | — | — |
| TempleOS | Terry A. Davis | 2013 | — |
| TENEX | BBN Technologies | 1969 | — |
| The Linux Schools Project | — | — | — |
| THE multiprogramming system | Eindhoven University of Technology | — | — |
| Theseus | — | — | MIT License |
| Thinstation | — | — | GNU General Public License |
| Thoth (operating system) | University of Waterloo | — | — |
| ThreadX | — | — | MIT License |
| Tilix | — | — | — |
| Time Sharing Operating System | RCA Corporation | 1968 | — |
| Time Warp Operating System | Jet Propulsion Laboratory | — | — |
| Tin Hat Linux | — | — | — |
| Tinfoil Hat Linux | — | — | BSD licenses |
| Tiny Core Linux | — | — | GNU General Public License |
| TinyKRNL | — | — | — |
| TinyMe | — | — | — |
| TinyOS | — | — | BSD licenses |
| Tisa Linux | — | — | — |
| Tizen | Intel, Linux Foundation, Samsung Group, Tizen Association | — | Apache Software License 2.0, Flora License |
| TI‐RTOS | Texas Instruments | — | BSD licenses |
| ToaruOS | K Lange | — | University of Illinois/NCSA Open Source License |
| Tock | Brad Campbell, Amit Levy | — | Apache Software License 2.0, MIT License |
| Tomato | — | — | GNU General Public License |
| Tomsrtbt | — | — | — |
| TopologiLinux | — | — | GNU General Public License |
| TOPS-10 | Digital Equipment Corporation | — | — |
| TOPS-20 | Leonard Bosack, Daniel Murphy, Digital Equipment Corporation | 1976 | — |
| Tor-ramdisk | — | — | GNU General Public License, version 3.0 |
| Toutankinux | — | — | — |
| Toutou Linux | — | — | — |
| Towns OS | — | — | — |
| TR-DOS | — | — | — |
| Transaction Management eXecutive | NCR Corporation | — | — |
| Transaction Processing Facility | IBM | 1979 | — |
| Tri-Lab Operating System | — | — | — |
| Trinity Rescue Kit | — | 2010 | GNU General Public License |
| Trinux | — | — | GNU General Public License, version 2.0 |
| TRIPOS | MetaComCo, University of Cambridge | 1978 | — |
| Trisquel | — | 2007 | GNU General Public License, version 3.0 |
| Trix | MIT Computer Science and Artificial Intelligence Laboratory | — | — |
| Tropix | Federal University of Rio de Janeiro | — | — |
| TRSDOS | Tandy Corporation | — | — |
| Tru64 UNIX | Hewlett-Packard, Digital Equipment Corporation, IBM, Compaq | — | proprietary license |
| TrueNAS | IXsystems | — | proprietary license, GNU General Public License, version 3.0, BSD licenses |
| TrueOS | IXsystems, Kris Moore | 2006 | BSD licenses |
| Trusted Solaris | Sun Microsystems | — | — |