Siegbahn, who was born at Örebro in Sweden, was educated at the University of Lund, where he studied astronomy, mathematics, physics, and chemistry, obtaining his doctorate in 1911. In 1914 he turned his attention to the new science of x-ray spectroscopy. It had already been established from x-ray spectra that there were two distinct ‘shells’ of electrons within atoms, each giving rise to groups of spectral lines, labeled ‘K’ and ‘L’. In 1916 Siegbahn discovered a third, or ‘M’, series. (More were to be found later in heavier elements.)

Through successive refinement of his x-ray equipment and technique, Siegbahn was able to achieve a significant increase in the accuracy of his determinations of spectral lines. This allowed him to make corrections to Bragg's equation for x-ray diffraction to allow for the finer details of crystal diffraction. Besides working with crystals, he performed x-ray spectroscopy at longer wavelengths using gratings. Here again his accurate measurements revealed discrepancies that were later shown to result from inaccuracies in the value assumed for the electronic charge.

In 1920 Siegbahn was made professor and head of the physics department at the University of Lund and in 1923 he moved to the University of Uppsala to become chairman of the physics department. In 1924 he received the Nobel Prize for physics, cited for “his discoveries and research in the field of x-ray spectroscopy,” and the following year saw publication of his influential book Spectroscopy of X-rays (1925). In the same year Siegbahn and his colleagues showed that x-rays are refracted as they pass through prisms, in the same way as light.

When, in 1937, the Swedish Royal Academy of Sciences created the Nobel Institute of Physics at Stockholm, Siegbahn was appointed its first director. In the same year he became professor of physics at the University of Stockholm, retaining this post until his retirement in 1964. He was responsible for the building of accelerators, laboratory spectrometers, and other equipment at the Nobel Institute.