C9

C9, C09 or C-9 may refer to:
* C9, an international standard paper size (40×57 mm), defined in ISO 216.
* C9 engine, by Caterpillar Inc.
* C9, a global group of nine leading economists based in Zürich, Switzerland
* C9, an extremely high "musical" note five octaves above Middle C. Only Georgia Brown (Brazilian singer) (not even Mariah Carey, Minnie Riperton or Adam Lopez) is reputed to sing this high
* C9, a subsidiary of Champion sportswear company sold exclusively at Target stores
* ATC code C09, a section of the Anatomical Therapeutic Chemical Classification System
* C09, Malignant neoplasm of tonsil ICD-10 code

Transportation

* Cierva C.9, a 1927 British experimental autogyro
* HMS "C9", a C-class submarine of the Royal Navy
* McDonnell Douglas C-9 (C-9A Nightingale/C-9B Skytrain II), a transport aircraft of the United States Air Force (Nightingale) or U.S. Naval Reserve and United States Marine Corps (Skytrain II). It is the military version of McDonnell Douglas DC-9
* USS "Montgomery" (C-9), a protected cruiser of the United States Navy
* C9, the IATA code for Cirrus Airlines

Weapons

* Hi-Point Models C9 and C9 Comp handguns
* C9 LMG, a Canadian variant of the FN MINIMI light machine gun