| Damn Small Linux | John Andrews | — | GNU General Public License |
| Damn Vulnerable Linux | — | — | — |
| Darik's Boot and Nuke | — | — | GNU General Public License |
| DarkNess reDefined | — | 2017 | — |
| Darkstar Linux | — | — | — |
| Dartmouth Time Sharing System | Dartmouth College | — | — |
| Darwin | Apple Inc. | 2000 | Apple Public Source License |
| Data General AOS | Data General | — | — |
| DAVIX | — | — | — |
| Dawn | — | — | — |
| Daylight Linux | — | — | — |
| DC/OSx | Pyramid Technology | — | — |
| DD-WRT | — | — | GNU General Public License |
| Debian | The Debian Project | 1993 | GNU General Public License |
| Debian Edu | — | — | GNU General Public License |
| Debian Etch | — | — | — |
| Debian GNU/Hurd | — | — | GNU General Public License |
| Debian GNU/kFreeBSD | — | — | GNU General Public License |
| Debian GNU/NetBSD | — | — | — |
| Debian Hamm | — | 1998 | — |
| Debian Lenny | — | — | — |
| Debian Live | The Debian Project | — | — |
| Debian Med | — | — | — |
| Debian Potato | — | 2000 | — |
| Debian Pure Blend | — | — | — |
| Debian Sarge | — | — | — |
| Debian Slink | — | 1999 | — |
| Debian Squeeze | — | — | — |
| Debian Wheezy | — | — | — |
| Debian Woody | — | — | — |
| DebWRT | — | — | — |
| DEC BATCH-11/DOS-11 | Digital Equipment Corporation | — | — |
| Deepin | — | 2016 | GNU General Public License |
| DEFT Linux | Stefano Fratepietro | — | GNU General Public License |
| DeLi Linux | — | — | — |
| DEMOS | National Research Centre Kurchatov Institute | — | BSD licenses |
| DerrickOS | Jozef Bogin | — | — |
| Desinfec't | — | — | — |
| DesktopBSD | — | — | BSD licenses |
| DESQview | Quarterdeck Office Systems | — | — |
| Deutsche Linux-Distribution | Dirk Haaga | — | — |
| Devil-Linux | — | — | — |
| Devuan | Dyne foundation | — | GNU General Public License |
| DexOS | — | — | — |
| DG/UX | Data General | — | — |
| DGA4131 FW_248 | — | — | GNU General Public License, version 2.0 |
| DICOS | Ericsson | — | — |
| DietPi | — | 2014 | — |
| DigitaOS | — | 1998 | — |
| DioneOS | — | — | — |
| DIP-DOS | — | — | — |
| Dirty Unicorns | — | 2012 | GNU General Public License, version 2.0, GNU General Public License, version 3.0 |
| DiskStation Manager | Synology Inc. | — | — |
| Distro Astro | — | — | — |
| DivestOS | — | — | — |
| DNALinux | — | — | — |
| DNIX | Dataindustrier AB | — | — |
| Dolphin OS | — | — | — |
| Domain/OS | Apollo Computer | 1981 | — |
| DOS Plus | Digital Research | — | — |
| DOS/360 | IBM | 1966 | — |
| DOSB10 | — | — | — |
| DotOS | — | 2017 | — |
| DoudouLinux | — | — | — |
| Dpup | — | — | — |
| DR-DOS | DRDOS Inc, Digital Research | — | — |
| Draco GNU/Linux | Ole-André Rodlie | — | 3-clause BSD License, GNU Lesser General Public License, version 2.1, GNU General Public License, version 2.0 |
| Dracos | — | — | — |
| DragonFly BSD | Matthew Dillon | 2003 | BSD licenses |
| Dragora GNU/Linux-libre | — | — | GNU General Public License |
| Dreamcast system software | Sega | — | — |
| Dreamlinux | — | — | GNU General Public License |
| DreamShell | — | — | BSD licenses |
| Droidian | — | — | — |
| DRYOS | Canon Inc. | — | proprietary license |
| DSLinux | — | — | GNU General Public License |
| DSOS | Texas Instruments | — | — |
| DuinOS | — | — | GNU General Public License |
| Dusk OS | — | — | GNU General Public License, version 3.0 |
| DuZeru | — | — | — |
| DX-DOS | — | — | — |
| DX10 | Texas Instruments | — | — |
| Dynebolic | — | — | GNU General Public License |
| Dynix | Sequent Computer Systems, IBM | — | — |
| Dyson | — | 2012 | — |