A nephelometer (from the Greek νεφέλη, cloud, and μέτρον, measurement)[1]​ is an instrument for measuring particles suspended in a liquid or a gas, for which it uses a photocell placed at an angle of 90° with respect to a light source. "Particle density"), or particle content per unit volume, is related to the intensity of light scattered by the particles in suspension towards the photocell.

Nephelometers used in water quality testing are commonly called turbidimeters. However, there may be differences between turbidimeter models, depending on the geometric configuration of the light source with respect to the photocell. A nephelometric turbidimeter always detects light scattered by particles and not attenuation due to turbidity.

The turbidity unit for a calibrated nephelometer is called Nephelometric Turbidity Unit, UTN or NTU.