In automatic control, a regulator is a device which has the function of maintaining a designated characteristic. It performs the activity of managing or maintaining a range of values, in a machine. The measurable property of a device is managed closely by specified conditions or an advance set value; or it can be a variable according to a predetermined arrangement scheme. It can be used generally to connote any set of various controls or devices for regulating or controlling items or objects.

Examples are a voltage regulator (which can be a transformer whose voltage ratio of transformation can be adjusted, or an electronic circuit that produces a defined voltage), a gas regulator which maintains its output at a fixed pressure lower than its input, and a fuel regulator (which controls the supply of fuel).

In social science, a regulator is an official responsible for control and supervision of an activity or area of public interest.

In algebraic number theory, the regulator of a number field is a quantity defined from the group of units. After taking logarithms, it has the interpretation of the volume of a fundamental domain in the lattice of units in 'logarithmic space'.

In the British Royal Navy, a Regulator is a naval policeman (or woman), a member of the Royal Navy Regulating Branch.

Regulator is an also old Southern United States term for vigilante.