The Secret Intelligence Service (SIS) of the United Kingdom is an agency that mainly deals in foreign intelligence. Formerly known as MI6 (Military Intelligence Section 6). Headquartered in London, the organization is classified as a branch of the Foreign and Commonwealth Office, but is under the direct jurisdiction of the Prime Minister.

Liquid Snake once served in the SIS. He operated behind the scenes of the political arena, engaging in top-secret wetwork, or assassinations.