9-9-9 is the United Kingdom's Emergency telephone number along with the EU standard 1-1-2. Either number can be used but people always refer to 9-9-9.

It is also the emergency telephone number in some Commonwealth counties. It is an all-service number, meaning that it should be called in any situations where state run emergency services are needed. The three main and best known services are fire brigade, police and ambulance. Other available services include coastguards, mountain rescue and cave rescue (where locally relevant). Some situations such as a major car accident or a terrorist attack (including nuclear, biological, or chemical attacks) will require multiple and/or specialist services but the first point of call for reporting such incidents from the general public is still the 9-9-9 system. The number is operated by British Telecom for the Home Office.

In some situations there will be specific instructions on nearby signs to notify some other authority of an emergency before calling 9-9-9. For example there are notices on bridges carrying railways over roads telling people that, if they see a road vehicle striking the bridge, they should call the railway authority (on a given number) first and then call 9-9-9 to inform the police.

It is also used in the Republic of Ireland