OpenSolaris is an open source operating system based on Sun Microsystems' Solaris. It is also the name of the project initiated by Sun to build a developer and user community around it.

OpenSolaris is derived from the Unix System V Release 4 codebase, with significant modifications made by Sun since it bought the rights to that code in 1994. It is the only open source System V derivative available. Open sourced components are snapshots of the latest Solaris release under development. Sun has announced that future versions of its commercial Solaris operating system will be based on technology from the OpenSolaris project.