CatOS (short for Catalyst Operating System) is a somewhat antiquated operating system for Cisco's Catalyst switch line obtained after Cisco bought Crescendo Communications. CatOS is rapidly being supplanted by the more commonly used Cisco IOS, and will reach EOL in 2009; at this point, there will be no further releases, bug fixes, or other maintenance to the operating system.. CatOS notably only operates on Layer 2, and requires a hybrid IOS/CatOS installation in order to have both Layer 2 and Layer 3 capability operating on the same device.

This OS is occasionally championed for it's VLAN-management interfaces, and is occasionally still found in existence on various species of the Catalyst 4xxx and 65xx switches. The latter these formerly shipped with only CatOS installed. Recommended best practice for operation before the release of the SUP 720 blade was to run CatOS and IOS in the aforementioned hybrid mode; this has become much less common in the last three years since the EOL announcement.