C9 or C-9 may refer to:

• C9, an international standard paper size (40×57 mm), defined in ISO 216.
• C9, the IATA code for Cirrus Airlines.
• C9, the Canadian (and a few other countries') designation of the FN M249 machine gun.
• The C-9 Nightingale, a transport aircraft of the United States Air Force.
• The C-9 Skytrain II, a military version of the McDonnell Douglas DC-9.
• HMS C9, a C-class submarine of the Royal Navy.
• USS Montgomery (C-9), a protected cruiser of the United States Navy.
• C9 engine, by Caterpillar Inc.
• C9 is a group of nine Swiss Economists based in Zurich, Switzerland.
• C9, an extremely high "musical" note five octaves above Middle C. Only Georgia Brown (not even Mariah Carey, Minnie Riperton or Adam Lopez) is reputed to sing this high.
• CloudNine (C9) Web Hosting, an internet company. CloudNine