| S. | Signals. |
| S.A. | Semi-automatic; supply assistant (R.N.). |
| S.A.A. | Small arms ammunition. |
| S.A.A.A. | School of Anti-Aircraft Artillery. |
| S.A.A.F. | South African Air Force. |
| S.A.C.S.E.A. | Supreme Allied Commander, South-East Asia. |
| S.A.D.F. | South African Defence Force. |
| S. & T. | Supply and Transport. |
| S.A.N.S. | South African Naval Service. |
| S.A.P. | Semi armour-piercing. |
| S.A.R. | School of Aircraft Recognition. |
| S.A.S. | School of Air Support; Special Air Service. |
| S.A.S.O. | Senior Air Staff Officer. |
| S.B. | Signal boatswain; stretcher-bearer. |
| S.B.A. | Sick-bay attendant. |
| S.C. | Small craft; Staff Captain; Staff College. |
| S.C.C. | Sea Cadet Corps. |
| S.C(dr). | Senior Commander (A.T.S.). |
| S.C.F. | Senior Chaplain to the Forces. |
| S.D. | Service dress; special duties; staff duties; supply dropping. |
| S.D.O. | Senior Dental Officer; Signal Distributing Office. |
| S/E. | Single-engined. |
| S.E.A.A.F. | South-East Asia Air Forces. |
| S.E.A.C. | South-East Asia Command. |
| S.E.A.L.F. | South-East Asia Land Forces. |
| Sec.Lt. | Second-Lieutenant. |
| S.Eq.S.O. | Senior Equipment Staff Officer. |
| S.F.P. | Supplementary Fire Party. |
| S.F.T.S. | Service Flying Training School or Squadron. |
| S.G. | Scots Guards; seaman gunner. |
| Sg.C. | Surgeon Captain (R.N.). |
| Sgmn. | Signalman. |
| Sg.R.A. | Surgeon Rear-Admiral. |
| Sgt. | Sergeant. |
| S.H.A.E.F. | Supreme Headquarters, Allied Expeditionary Force. |
| Sh.L. | Shipwright Lieutenant. |
| S.H.Q. | Squadron Headquarters; Station Headquarters. |
| S.I.B. | Special Investigation Branch. |
| Sig.L. | Signals Lieutenant (R.K). |
| Sigs. | Signals. |
| S.I.O. | Senior Intelligence Officer (R.A.F.). |
| Sitrep. | Situation report. |
| Sjt. | Serjeant. |
| S.L. | Searchlight; Squadron Leader. |
| S.L.C. | Searchlight control (radar). |
| S/Lt. | Sub-Lieutenant. |
| S.M. | Sergeant-Major. |
| S.M.L.E. | Short muzzle Lee Enfield (rifle). |
| S.M.O. | Senior Medical Officer. |
| S.N.L.R. | Services no longer required. |
| S.N.O. | Senior naval officer; senior navigating officer. |
| S.O. | Section Officer (W.A.A.F.); Signals Officer, Staff Officer. |
| S.O.P. | Sleeping-out pass. |
| S.O.R. | Sector Operations Room. |
| S.O.R.S. | Struck off ration strength. |
| S.O.S. | Struck off strength. |
| Sp. | Support. |
| S.P. | Self-propelled; Service Police; staging post; starting point; stirrup pump; stretcher party. |
| S.P.O. | Stoker petty officer. |
| Spr. | Sapper. |
| S.P.S.O. | Senior Personnel Staff Officer. |
| S.Q.M.S. | Staff Quartermaster-Sergeant. |
| Sqn. | Squadron. |
| Sqn. Ldr. | Squadron Leader. |
| Sq.O. | Squadron Officer (W.A.A.F.). |
| S.R.E. | Scientific Research and Experiments Department (R.N.). |
| S.R.H. | Supply railhead. |
| S.S. | Services of Supply (U.S.). |
| S.S.A.F.A. | Soldiers', Sailors' and Airmen's Families Association. |
| S/Sgt. | Staff Sergeant. |
| S.S.M. | Squadron (or Staff) Sergeant-Major. |
| S.S.O. | Station Staff Officer. |
| S.S.Q. | Station Sick Quarters. |
| Stalag. | Stammlager (Ger.), other ranks' prison camp. |
| S.T.C. | Senior Training Corps. |
| Sto. | Stoker. |
| S.To. | Seaman Torpedornan. |
| S.T.O. | Sea Transport Officer. |
| Strat. | Strategic. |
| Sub. | Subaltern; submarine. |
| Subs. | Substantive. |
| Sup. | Supply. |
| S.V. | Sailing vessel. |
| Svy. | Survey. |
| S.W. | Shelter Warden. |
| S.W.B. | South Wales Borderers. |
| S.W.O. | Squadron (or Station) Warrant Officer. |
| Sy.P.O. | Supply Petty Officer. |