Kai Siegbahn, Swedish physicist and academic, Nobel Prize laureate (died 2007)

Kai Manne Börje Siegbahn was a Swedish physicist who shared the 1981 Nobel Prize in Physics.

The Nobel Prize in Physics is an annual award given by the Royal Swedish Academy of Sciences for those who have made the most outstanding contributions to mankind in the field of physics. It is one of the five Nobel Prizes established by the will of Alfred Nobel in 1895 and awarded since 1901, the others being the Nobel Prize in Chemistry, Nobel Prize in Literature, Nobel Peace Prize, and Nobel Prize in Physiology or Medicine. Physics is traditionally the first award presented in the Nobel Prize ceremony.