Seth Neddermeyer (1907-1988) was an American physicist.
Neddermeyer was recruited to work on the Manhattan Project by J. Robert Oppenheimer from the National Bureau of Standards. He proposed using an implosion method for the bomb that would use powerful explosives to compress a core of radioactive plutonium to a critical mass. Although Oppenheimer doubted the feasibility of the implosion method, he appointed Neddermeyer to the new post of group leader in the Ordnance Division for implosion experimentation.
However, Neddermeyer struggled with the idea's implementation, and in January 1944 James B. Conant recruited George Kistiakowsky to help with the program. In June of that year, Oppenheimer asked Kistiakowsky to assume all work on implosion as an associate division leader and demoted Neddermeyer to senior technical adviser. The implosion design would be used in the Trinity Test and Fat Man bomb.