Teller testified at the security hearing of J. Robert Oppenheimer, the wartime director of Los Alamos. While not explicitly accusing Oppenheimer of disloyalty, Teller stated that he would 'personally feel more secure if public matters would rest in other hands.' This testimony contributed to the revocation of Oppenheimer's security clearance.

Teller's testimony at the Oppenheimer hearing led to widespread criticism from many in the scientific community. He was ostracized by some colleagues and viewed with suspicion for the rest of his career. The controversy deeply affected his personal and professional life.