Sylvester James Gates Jr. (born December 15, 1950), known as S. James Gates Jr. or Jim Gates, is an American theoretical physicist who works on supersymmetry, supergravity, and superstring theory. He is currently the Toll Professor of Physics at the University of Maryland. He also holds the Clark Leadership Chair in Science with the physics department at the University of Maryland College of Computer, Mathematical, and Natural Sciences. He is also affiliated with the University Maryland's School of Public Policy. He previously was the Brown University Theoretical Physics Center Director and the Ford Foundation Professor of Physics. He served on former president Barack Obama's Council of Advisors on Science and Technology.

Gates, the oldest of four siblings, was born in Tampa, Florida, the son of Sylvester James Gates Sr., a career U.S. Army man, and Charlie Engels Gates. His mother died at age 44 of breast cancer when he was 11. Gates, Sr. raised his children while serving full-time in the U.S. Army and retired as a sergeant major after 27 years of service — one of the first African Americans to earn this position. Gates, Sr., later worked in public education and as a union organizer.

Both of Gates' parents were extraordinarily committed to their children's educations, though neither had the opportunity to go to college. Gates, Sr. never finished high school, as he enlisted in the U.S Army at the age of seventeen. He later earned a G.E.D. The family moved many times while Gates was growing up, but, in January 1963, settled in Orlando, Florida, where Gates Jr. attended Jones High School—his first experience in a segregated African-American school. Comparing his own school's quality to neighboring white schools, "I understood pretty quickly that the cards were really stacked against us." Nevertheless, an 11th grade course in physics established Gates' career interest in physics, especially its mathematical side. At his father's urging, he applied for admission to MIT.

Gates received two B.S. degrees from MIT in mathematics and physics (1973), as well as his Ph.D. (1977). For his undergraduate thesis he wrote On the Feasibility of Generating Electricity with a Rijke Tube. His doctoral thesis, under the mentorship of James E. Young, was the first at MIT on supersymmetry. With M. T. Grisaru, M. Rocek and W. Siegel, Gates coauthored Superspace, or One thousand and one lessons in supersymmetry (1984), the first comprehensive book on supersymmetry.

Gates has taught every year since 1972. After his graduation from MIT in 1977, Gates accepted a Junior Fellowship at Harvard, the first Black scientist to be appointed a Junior Fellow. He remained at Harvard until 1980, when he accepted a postdoctoral research appointment at CalTech, which lasted until 1982. He was the first Black postdoctoral researcher to be appointed in CalTech's Division of Physics, Mathematics, and Astronomy at Caltech. In 1982, he returned to MIT as an assistant professor of physics. In 1984, he gained an associate professor position at the University of Maryland (UMD) physics department. Four years later, he became a full professor of physics at UMD, a position he held until 2017, becoming the first African American to have an endowed position in physics at a major American research university.

In 1990, Gates was invited to serve as the chair of an external committee to evaluate the physics department at Howard University. After he submitted the committee's report to the dean, he was asked to join Howard as the chair of the physics department. He accepted and took a leave of absence from UMD from 1991-1993 to serve as chair of the department at Howard. While there, he led the creation of a new NASA-funded research center, called the Center for the Study of Terrestrial and Extraterrestrial Atmospheres (CSTEA), of which he served as the first director.

Gates returned to UMD in 1993 and remained there until his retirement in 2017. Just prior to that, he spent a year at Dartmouth as the Roth Distinguished Scholar from 2015-2016. In 2017, he retired from UMD as an emeritus professor and joined Brown University as the Director of the Brown Theoretical Physics Center, the Ford Foundation Professor of Physics, an Affiliate Mathematics Professor, and a Faculty Fellow in the Watson Institute for International Studies & Public Affairs. In 2022 Gates rejoined the University of Maryland as the John S. Toll Professor of Physics, Clark Leadership Chair in Science in the department of physics and the school of public policy at the University of Maryland.

Gates is on the board of trustees of Society for Science & the Public and is active in scientific outreach.