C1 or C-1 may refer to:
- In vehicles
- C1 Corvette, any model corvette sold from 1953 to 1962, denoted a solid-axle rearend
- BMW C1, a security scooter with a roof
- Citroën C1, a small car
- Ford C1 platform, the Ford Motor Company's global compact car automobile platform
- C1, a British designation for a cargo/transport aircraft
- Kawasaki C-1, an indigenous transport aircraft of the Japanese Air Self-Defense Forces
- C-1 Trader, a cargo-version of the S-2 Tracker anti-submarine warfare aircraft of the United States Navy
- Douglas C-1, a cargo/transport airplane produced by Douglas Aircraft for the U.S. Army Air Service starting in 1925
- HMS C1, a C-class submarine of the Royal Navy
- USS C-1 (SS-9), a C-class submarine of the United States Navy
- USS Newark (C-1), a protected cruiser of the United States Navy
- AEG C.I, a German World War I reconnaissance aircraft
- 'C1' was the top category in Group C sportscar racing, used between 1985 and 1990
- In computing
- C1 (protocol), a protocol for file transfer, also known as "Punter"
- C0 and C1 control codes defined in ISO 6429
- Counter-Strike "C1", a slang expression from the video game, used to express agreement
- TCSEC C1 security class
- C-One single-board microcomputer
- ATI C1, codename of the Xbox 360 GPU "Xenos"
- In sports
- C1 is an abbreviation for single-person Sprint Canoe.
- In football, C1 refers to the UEFA Champions League (formerly known as the European Cup)
- In other uses
- ISO 216 C1, an international standard paper size (648×917 mm)
- NRS social grade C1 for the United Kingdom is the lower middle class
- Cytochrome C1, a protein, precursor to Cytochrome C
- First cervical vertebra of the vertebral column, in human anatomy
- note-octave C1 in music
- In town and country planning in the UK, C1 is the code for permission to use specific land or premises for hotels
- In mathematics, the notation <math>C^1</math> refers to the set of all continuously differentiable functions
- In biochemistry, C1 refers to small molecules with one carbon atom, such as methane, methanol, formaldehyde and formate, carbon dioxide, and carbon monoxide.
- C1 Television, a Mongolian Television Channel
- 'C1' is the numberplate of the Prime Minister of Australia's official VIP transportation.
- %C1 is the Url Encoded version of the Acute accent for A (Á)
- C1 is a level in the Common European Framework of Reference for Languages
- A unit of the South African Police responsible for assassinating opponents of Apartheid during the 1980s (also known as Vlakplaas).
