The Stark effect is the broadening or splitting of a spectral line that results when an electric field slightly changes the energy levels of a radiating atom or ion. Stark broadening is proportional to the ion and electron density in a plasma and is therefore a good indicator of pressure in a stellar atmosphere and hence of the star's luminosity. The effect is named after the German physicist Johannes Stark who discovered it in 1913.

The Stark effect follows from the electron theory of Hendrik Lorentz and is the electrical analogue of the Zeeman effect.