S9

S9 can be:
* S9 (Berlin), a S-Bahn line
* S9 fraction, a postmitochondrial liver fraction
* , a safety phrase in chemistry
* S9 postcode, covering areas of north eastern Sheffield
* British NVC community S9, a swamps and tall-herb fens community in the British National Vegetation Classification system
* County Route S9 (California)
* Motorola S9, a Bluetooth stereo headset
* Prussian S 9 (Experimental), a 1904 steam locomotives class
* SIAI S.9, a 1918 Italian flying boat
* USS S-9 (SS-114), a 1920 S-class submarine of the United States Navy
* East African Safari Air IATA airline designator
* A slang term used in CB radio to refer to excellent reception. Also, "S9" was a magazine published in the early 1960s to early 1980s in the United States devoted to CB radio
* Sometimes the filename extension for Motorola S-records
* a Rhein-Ruhr S-Bahn line
* a Rhine-Main S-Bahn line
* a Zürich S-Bahn line
* a Stadtbahn Karlsruhe line

S09 may refer to :
* HMS Oberon (S09), a 1961 British Royal Navy Oberon-class submarine
* Brave Fire S09, a Osamu Tezuka anime

S₉ may refer to :
* a cyclic allotrope of sulfur