C1 or C-1 may refer to:

• The Hawker-Siddeley C1 Andover transport aircraft.
• The C-1 Trader, an anti-submarine warfare aircraft of the United States Navy.
• Cytochrome C1, a protein, precursor to Cytochrome C.
• The Ford C1 platform, the Ford Motor Company's global compact car automobile platform.
• C1, a protocol for file transfer, also known as "Punter".
• "C1", a slang expression from the video game Counter-Strike, used to express agreement.
• HMS C1 , a C-class submarine of the Royal Navy.
• USS C-1, a C-class submarine of the United States Navy.
• USS Newark (C-1), a protected cruiser of the United States Navy.
• C1, an international standard paper size (648×917 mm), defined in ISO 216.