OpenSolaris () is a discontinued open-source computer operating system based on Solaris and created by Sun Microsystems. It was also, perhaps confusingly, the name of a project initiated by Sun to build a developer and user community around the eponymous operating system software.

OpenSolaris is a descendant of the UNIX System V Release 4 (SVR4) code base developed by Sun and AT&T in the late 1980s and is the only version of the System V variant of UNIX available as open source. OpenSolaris was developed as a combination of several software consolidations that were open sourced starting with Solaris 10. It includes a variety of free software, including popular desktop and server software.

After Oracle’s acquisition of Sun Microsystems in 2010, Oracle discontinued development of OpenSolaris in house, pivoting to focus exclusively on the development of the proprietary Solaris Express (now Oracle Solaris).

Prior to Oracle's close-sourcing Solaris, a group of former OpenSolaris developers began efforts to fork the core software under the name OpenIndiana. The illumos Foundation, founded in the wake of the discontinuation of OpenSolaris, continues to develop and maintain the kernel and userland of OpenIndiana (together renamed “illumos”), while the OpenIndiana Project (now under the auspices of the illumos Foundation) continues to maintain and develop the illumos-based OpenIndiana distribution (including its installer and build system) as the direct descendant of OpenIndiana. Since then additional illumos distributions, both commercial and non-commercial, have appeared and are under active development, combining the illumos kernel and userland with custom installers, packaging and build systems, and other distribution-specific utilities and tooling.