Heinrich Gustav Johannes Kayser ForMemRS[1] (German: [ˈkaɪzɐ]; 16 March 1853 – 14 October 1940) was a German physicist and spectroscopist.

Kayser's early work was concerned with the characteristics of acoustic waves.

[3] He discovered the occurrence of helium in the Earth's atmosphere in 1868 during a solar eclipse when he detected a new spectral line in the solar spectrum.

Together with Carl Runge, he examined the spectra of chemical elements.

[7] The kayser unit, associated with wavenumber, of the CGS system was named after him.