A cotter is a pin or wedge passing through a hole to fix parts tightly together. In British usage cotter pin has the same meaning, but in the U.S. it refers to a different fastener.

Cotter pin may refer to:
In U.S. usage:

• Split pin, a metal fastener with two tines that are bent during installation used to fasten metal together, like with a staple or rivet
• Hairpin cotter pin, more commonly known as an "R-clip"
• Bowtie cotter pin, a vibration-proof type of R-clip that is shaped like a bowtie
• Circle cotter, a ring-shaped cotter pin