William James Perry (born October 11, 1927) is an American businessman and engineer who was the United States Secretary of Defense from February 3, 1994, to January 23, 1997, under President Bill Clinton. He had been Deputy Secretary of Defense from 1993 to 1994.

Born in Vandergrift, Pennsylvania, he graduated from Butler High School in 1945 and served in the United States Army as an enlisted man from 1946 to 1947, including service in the Occupation of Japan. Perry later received a commission in the United States Army Reserve through ROTC, serving from 1950 to 1955. He received his B.S. (1949) and M.A. (1950) degrees from Stanford University, and a Ph.D. in mathematics from Pennsylvania State University in 1957. He was director of the Electronic Defense Laboratories of Sylvania/GTE in California from 1954 to 1964, and from 1964 to 1977 president of ESL, Incorporated, an electronics firm that he helped found. From 1977 to 1981, during the Jimmy Carter administration, Perry served as undersecretary of defense for research and engineering, where he had responsibility for weapon systems procurement and research and development. Among other achievements, he was instrumental in the development of stealth aircraft technology.

On leaving The Pentagon in 1981 Perry became managing director until 1985 of Hambrecht & Quist, a San Francisco investment banking firm that specialized in high technology companies. Later in the 1980s and up to 1993, before returning to the Pentagon as deputy secretary of defense, he held positions as chairman of Technology Strategies Alliances, professor in the School of Engineering at Stanford University, and a co-director of the Preventive Defense Project at Stanford's Center for International Security and Cooperation.