Sir John Alexander Sinclair, KCMG, CB, OBE (29 May 1897 – 22 March 1977) was a British Army general who was head of the Secret Intelligence Service (SIS) from 1953 to 1956.

[8] Following his retirement from the military in 1952 as a Major-General,[7] Sinclair was appointed head of the SIS, taking up the post in 1953.

[12] Sir John's retirement coincided with a failed frogman mission to investigate the Sverdlov-class cruiser Ordzhonikidze that had brought the leader of the Soviet Union, Nikita Khrushchev, and Prime Minister Nikolai Bulganin on a diplomatic mission to Britain, resulting in the death of frogman Lionel Crabb.

The Prime Minister had not approved this mission and some accounts incorrectly claimed that Sir John had been forced to resign.

[13] The "Authorized History of MI5" confirms that the decision that the head of that service should succeed Sir John at his planned retirement date in 1956 had been taken by the Prime Minister in 1954.