Which version of Mac OS X was the first to be a free upgrade from the previous version?

OS X Mavericks was the first version of Mac OS X to be offered as a free upgrade from the previous version. Apple released version 10.9 at no charge on October 22, 2013.

Earlier major upgrades were generally paid releases, even when distributed digitally through the Mac App Store. Mavericks changed that policy: users running Snow Leopard 10.6.8 or later could upgrade without paying. Apple continued the free-upgrade model with later releases, including Yosemite, El Capitan, and subsequent versions of macOS.

Mavericks also marked a naming shift. It was the first OS X release named after a California location rather than a big cat; the name refers to the Northern California surfing spot. The release focused on battery-life improvements, Finder enhancements, Tags, and tighter iCloud integration. It is easy to confuse Mavericks with OS X Lion or Mountain Lion because those were the first major releases distributed through the Mac App Store, but they were not free upgrades.