Which version of Mac OS X was the first to be certified as UNIX 03 compliant?

Mac OS X Leopard was the first version of Mac OS X to be certified as UNIX 03 compliant. The certification applied to Mac OS X 10.5 running on Intel-based Macintosh computers.

Leopard was certified by The Open Group under the Single UNIX Specification and became an officially registered UNIX 03 product in 2007. This was more than a marketing label: certification indicated that the system’s specified interfaces, commands, utilities, and behavior met the standard’s requirements for certified Unix systems.

The Intel qualification matters because UNIX 03 certification was tied to a particular product configuration, not automatically to every architecture on which Leopard ran. Leopard itself launched on October 26, 2007, supported both Intel and PowerPC Macs, and introduced major features such as Time Machine, Spaces, and a redesigned Finder. Snow Leopard and later releases inherited the Unix foundation, but they were not the first Apple systems to receive this certification.