Johannes (Janne) Robert Rydberg (Swedish: [ˈrŷːdbærj]; 8 November 1854 – 28 December 1919) was a Swedish physicist mainly known for devising the Rydberg formula, in 1888, which is used to describe the wavelengths of photons (of visible light and other electromagnetic radiation) emitted by changes in the energy level of an electron in a hydrogen atom.

[2] Rydberg's research was preceded by Johann Jakob Balmer's, who presented an empirical formula for the visible spectral lines of the hydrogen atom in 1885.

[6] To earn extra money he worked part-time as a numerical examiner at Sparbanken in Lund from 1891 and as an actuary in Malmö from 1905.

[11] Rydberg's anticipation that spectral studies could assist in a theoretical understanding of the atom and its chemical properties was justified in 1913 by the work of Niels Bohr (see hydrogen spectrum).

An important spectroscopic constant based on a hypothetical atom of infinite mass is called the Rydberg (R) in his honour.