G4S plc (formerly Group 4 Securicor) (LSE: GFS, OMX: G4S) is a global security services company headquartered in Crawley, United Kingdom. It is the world's largest security company measured by revenues and has operations in more than 125 countries. With over 630,000 employees, it is the world's second-largest private sector employer (after Wal-Mart Stores).

G4S has a primary listing on the London Stock Exchange and it is a constituent of the FTSE 100 Index. It has a secondary listing on the Copenhagen Stock Exchange.

G4S has its origins in a guarding business founded in Copenhagen in 1901 by Marius Hogrefe originally known as Kjøbenhavn Frederiksberg Nattevagt and subsequently renamed Falck.

In 2000 Group 4, a security firm formed in the 1960s in Belgium, merged with Falck to form Group 4 Falck. Following the decision by the British Government in 1993 to enter into a contract with Group 4 to provide security for prisons, the company was embarrassed after a series of security blunders, including escaped prisoners. In 2002 Group 4 Falck went on to buy Wackenhut. In 2003 Group 4 Falck signed a Detention Services Contract with the Department of Immigration and Multicultural and Indigenous Affairs (Australia). Under this contract Group 4 Falck took over direct control of Australia's refugee detention centres from Australasian Correctional Management, owned by its subsidiary Wackenhut, which had run them since 1998. In 2004 Group 4 Falck merged with Securicor, a security firm founded in 1935 in London, to form G4S.