A plethysmograph is an instrument for measuring changes in volume within an organ or whole body (usually resulting from fluctuations in the amount of blood or air it contains).

It is most commonly used to measure lung capacity. The test subject is typically placed inside a sealed chamber while performing breathing exercises. The difference in full versus empty lungs can be used to assess diseases and airway passage restrictions. Newer devices have an option which does not require enclosure in a chamber.

Some plethysmograph devices are attached to limbs and used to determine circulatory capacity.

Another common but more controversial type of plethysmograph is the penile plethysmograph. This device is used to measure changes in blood flow in the penis. Although some researchers use this device to assess sexual arousal and sexual orientation, the data are not generally considered scientifically reliable and are usually not admissible in court cases in the United States.

An approximate female equivalent to penile plethysmography is vaginal photoplethysmography.

Impedance plethysmography is a non-invasive method used to detect venous thrombosis in the legs or other areas of the body.