Alexander Suss Langsdorf Jr. (May 30, 1912 – May 24, 1996) was an American physicist on the team that developed the atomic bomb and several devices related to nuclear physics.

He earned an undergraduate degree from Washington University in St. Louis in 1932 and a doctorate in physics from Massachusetts Institute of Technology in 1937.

Prior to World War II, Langsdorf co-developed a cyclotron for splitting atomic particles at Washington University in St. Louis .

During World War II, he worked with Enrico Fermi at the University of Chicago on the Manhattan Project.

He helped found Bulletin of the Atomic Scientists, and his wife Martyl Langsdorf designed the 1947 cover of the publication which debuted the Doomsday Clock.