S11

S11, S-11, S.XI or S.11 may refer to:; In military :

* Fokker S-11, a two-seat aircraft used primarily as a flight trainer by the air forces of many countries
* HMS Orpheus (S11), a United Kingdom Royal Navy submarine commissioned in 1959
* SPAD S.XI, a French two-seat biplane reconnaissance aircraft of the First World War
* USS S-11 (SS-116), a 1921 S-class submarine of the United States Navy

; In transportation :

* S11 (Nissan Silvia), a car
* S11 (Rhine-Ruhr S-Bahn), a public transportation route in Germany
* County Route S11 (California), a road in the United States
* LSWR S11 class, a model of steam locomotive
* SABCA S-11, a 1931 Belgian aircraft, one prototype built
* a Hamburg S-Bahn line
* a Stadtbahn Karlsruhe line

; Other :

* S11 (protest), the protests at the meeting of the World Economic Forum in Melbourne, Australia on September 11, 2000.
* September 11, 2001 attacks
* S11 postcode, covering areas of south western Sheffield
* British NVC community S11, a swamps and tall-herb fens community in the British National Vegetation Classification system
* a S-Parameter in a 2-port Radio Frequency network

S₁₁ may refer to :
* a cyclic allotrope of sulfur