Kenneth Greisen was an American physicist. He worked on the Manhattan Project at Los Alamos. He was the leader of Group X-1A (Photogoraphy with Flash X-Rays) until it was dissolved in May 1945. He then became the leader of the Group X-7 (Detonator Developments). His wife, Elizabeth Greisen, accompanied him to Los Alamos during the war.
Greisen's speciality was the astrophysics of cosmic rays and gamma radiation. After the war, he was a professor of physics and astronomy at Cornell University.