James Franck (German: [ˈdʒɛɪ̯ms ˈfʁaŋk] ^ⓘ; 26 August 1882 – 21 May 1964) was a German–American atomic physicist who shared the 1925 Nobel Prize in Physics with Gustav Hertz "for their discovery of the laws governing the impact of an electron upon an atom".

Franck completed his doctorate in 1906 and his habilitation in 1911 at the Frederick William University in Berlin, where he lectured and taught until 1918, having reached the position of professor extraordinarius. He served as a volunteer in the German Army during World War I. He was seriously injured in 1917 in a gas attack and was awarded the Iron Cross 1st Class.

Franck became the head of the physics division of the Kaiser Wilhelm Gesellschaft for Physical Chemistry. In 1920, Franck became professor ordinarius of experimental physics and Director of the Second Institute for Experimental Physics at the University of Göttingen. While there he worked on quantum physics with Max Born, who was Director of the Institute of Theoretical Physics. His work included the Franck–Hertz experiment, an important confirmation of the Bohr model of the atom. He promoted the careers of women in physics, notably Lise Meitner, Hertha Sponer, and Hilde Levi.

After the Nazi Party came to power in Germany in 1933, Franck resigned from his position at the University of Göttingen in protest against the dismissal of fellow colleagues. He assisted Frederick Lindemann in helping dismissed Jewish scientists find work overseas, before he left Germany in November. After a year at the Niels Bohr Institute in Denmark, he moved to the United States, where he worked at Johns Hopkins University in Baltimore and then the University of Chicago. During this period he became interested in photosynthesis.

Franck participated in the Manhattan Project during World War II as director of the chemistry division of the Metallurgical Laboratory. He was also the Chairman of the Committee on Political and Social Problems regarding the atomic bomb, which is best known for the compilation of the Franck Report, which recommended that the atomic bombs not be used on the Japanese cities without warning.

James Franck was born on 26 August 1882 in Hamburg, Germany, into a Jewish family, the second child and first son of Jacob Franck, a banker, and Rebecca Nachum Drucker. He had an older sister, Paula, and a younger brother, Robert. His father was a devout and religious man, while his mother came from a family of rabbis.

In 1891, Franck enrolled at the Wilhelm-Gymnasium in Hamburg, which was then a boys-only school. Hamburg had no university then, so prospective students had to attend one of the 22 universities elsewhere in Germany. Intending to study law and economics, In 1901, Franck entered the University of Heidelberg, as it had a renowned law school. He attended lectures on law, but was far more interested in those on science. While there, he met Max Born, who would become a lifelong friend. With Born's help, he was able to persuade his parents to allow him to switch to studying physics and chemistry. He attended mathematics lectures by Leo Königsberger and Georg Cantor, but Heidelberg was not strong on the physical sciences, so he decided to go to the University of Berlin.

At Berlin, Franck attended lectures by Max Planck and Emil Warburg.. For his Ph.D., Warburg suggested that he study corona discharges. He found this topic too complex, so he changed the focus of his thesis. Titled Über die Beweglichkeit der Ladungsträger der Spitzenentladung (On the mobility of ions), it would subsequently be published in Annalen der Physik.