A sphincter is an anatomical structure, a circular muscle that normally maintains constriction of a natural body passage or orifice and which relaxes as required by normal physiological functioning. Sphincters are found in many animals; there are over 50 types in the human body, some microscopically small, in particular the millions of precapillary sphincters.

Sphincters prove effective in the mediation of the entrance or release of liquids and fluids; this is evident, for example, in the blowholes of numerous marine mammals.

Many sphincters are used every day in the normal course of digestion and vision. For example, the epiglottis is used to seal off the windpipe when swallowing, so as to ensure that no food or liquid enters the lungs. The function of the epiglottis is a typical example of an involuntary action by the body.

Sphincters can be further classified into functional and anatomical sphincters:

Sphincters can also be voluntarily or involuntarily controlled: