Kai Manne Börje Siegbahn (20 April 1918 – 20 July 2007) was a Swedish physicist who shared the 1981 Nobel Prize in Physics.

[2] He shared the 1981 Nobel Prize in Physics with Nicolaas Bloembergen and Arthur Schawlow.

Siegbahn received half the prize "for his contribution to the development of high-resolution electron spectroscopy" while Bloembergen and Schawlow received one quarter each "for their contribution to the development of laser spectroscopy".

In 1967 he published a book, ESCA; atomic, molecular and solid state structure studied by means of electron spectroscopy.

[1] At the time of his death he was still active as a scientist at the Ångström Laboratory at Uppsala University.