The Bikini state was an alert state indicator previously used by the UK Ministry of Defence to warn of non-specific forms of threat, including civil disorder, terrorism or war. Signs giving the current alert state were displayed at the entrance to government buildings and military installations. It was established on 19 May 1970. According to the Ministry of Defence, the word "bikini" was randomly selected by a computer.

Whilst similar to the DEFCON (defence readiness condition) alert states used in the United States, the Bikini levels were defined by the section of the military or organisation rather than UK-wide, and as a result, countermeasures and reactions to differing states may differ as acutely as from building to building. The highest levels of alert, Red and Amber, were only intended to be maintained for limited times. The White state has rarely been used, and is only known to have been used for periods between the Good Friday Agreement and 9 September 2001.

The system was illustrated in the British television drama Threads, produced by the BBC in 1984.

It was replaced by a more general and public terrorism alert status, the UK Threat Levels, an alert state system in use by the British government since 1 August 2006.