Marshall Holloway was an American physicist. He received his PhD in physics from Cornell University. In 1942, he directed a secret Manhattan Project assignment at Purdue University. The physicists at Purdue operated the university's cyclotron to determine cross sections for several molecules, which would prove key to developing the hydrogen bomb. In 1943, Holloway and his group at Purdue were transferred to Los Alamos.
At Los Alamos, Holloway worked on the Water Boiler, an aqueous homogeneous reactor. He also studied various safety aspects of the Little Boy bomb. He was involved in critical mass experiments for plutonium, and helped assemble the plutonium pit for "Gadget" for the Trinity test.
After the war, he served as Los Alamos representative and scientific director of the Operation Crossroads nuclear tests. He became head of Los Alamos National Laboratory's W Division, responsible for developing new weapons. In 1952, he was appointed director of the hydrogen bomb project. He later went on to work in industry.