.su was assigned as the country code top-level domain for the Soviet Union on September 19, 1990. It remains in use today, even though the Soviet Union itself no longer exists, and is administered by the Russian Institute for Public Networks (RIPN, or RosNIIROS in Russian transcription).

In 2001, the managers of the domain stated that they will commence accepting new .su registrations, but it is unclear whether this action is compatible with ICANN policies. ICANN has expressed intentions to terminate the .su domain, and the IANA states that the domain is being phased out, but lobbyists stated in September, 2007 that they had started negotiations with ICANN on retaining the domain. In the first quarter of 2008, .su registrations increased by 45%.

80829 domain names were registered by August 9, 2009.

Some famous Russian companies, having their roots in the USSR are still using this extension.