A proximity sensor is a type of sensor that can detect the presence of nearby objects without any physical contact. Proximity sensors emit an electromagnetic field or a beam of electromagnetic radiation, and look for changes in the field or return signal. The object being sensed is often referred to as the proximity sensors target. Different proximity sensor targets demand different sensors. For example, a capacitive proximity sensor or photoelectric sensor might be suitable for a plastic target, while an inductive proximity sensor always requires a metal target.

Proximity sensors can have a high reliability and long functional life because of the absence of mechanical parts and lack of physical contact between the sensor and the sensed object. They are also used in machine vibration monitoring to measure the variation in distance between a shaft and its support bearing, which is common in large steam turbines, compressors, and motors that use sleeve-type bearings.

Proximity sensors are commonly used on mobile devices. When the target is within nominal range, the device lock screen user interface will appear, thus emerging from what is known as sleep mode. Once the device has awoken from sleep mode, if the proximity sensors target is still for an extended period of time, the sensor will then ignore it.

There are many types of proximity sensors, and they each sense targets in distinct ways. The two most commonly used proximity sensors are the inductive proximity sensor and the capacitive proximity sensor. An inductive proximity sensor can only detect metal targets, while a capacitive proximity sensor can detect both metallic and non-metallic objects, including liquid, powders, and granular materials. Another type of proximity sensor is called a photoelectric proximity sensor, which can be reflective or through-beam.