Heinrich Gustav Johannes Kayser (16 March 1853 – 14 October 1940) was a German physicist.
He was born at Bingen am Rhein. Kayser's early work was concerned with the characteristics of acoustic waves. He discovered the occurrence of helium in the Earth's atmosphere. Together with Carl Runge, he examined the spectra of chemical elements. In 1905, he wrote a paper on electron theory.
The kayser unit, associated with wavenumber, of the CGS system was named after him. He died at Bonn in 1940.