Which Linux distribution is the basis for Google's Chrome OS? Gentoo is the answer.
ChromeOS is derived from ChromiumOS, Google's open-source project, and its system base was changed from Ubuntu to Gentoo Linux in February 2010. Google selected Gentoo largely because its Portage package-management system offered the flexibility needed for building a tightly controlled, appliance-like operating system.
ChromeOS is not simply an unmodified Gentoo installation. Google adds its own firmware, verified-boot system, Linux-kernel changes, browser-centered interface, update machinery, and hardware support. ChromeOS is the commercial product, while ChromiumOS is the publicly developed open-source counterpart.
A common mix-up comes from the Linux environment available inside ChromeOS. Linux applications run in a container whose default userland is Debian, so Debian is not the distribution underlying ChromeOS itself. Ubuntu was important during the project's early development, but Gentoo is the canonical answer for the established ChromeOS base.