James Harris Simons (April 25, 1938 – May 10, 2024) was an American hedge fund manager, investor, mathematician, and philanthropist. At the time of his death, Simons's net worth was estimated to be $31.4 billion, making him the 55th-richest person in the world. He was the founder of Renaissance Technologies, a quantitative hedge fund based in East Setauket, New York. He and his fund are known to be quantitative investors, using mathematical models and algorithms to make investment gains from market inefficiencies. Due to the long-term aggregate investment returns of Renaissance and its Medallion Fund, Simons was called the "greatest investor on Wall Street" and more specifically "the most successful hedge fund manager of all time".

Simons developed the Chern–Simons form (with Shiing-Shen Chern), and contributed to the development of string theory by providing a theoretical framework to combine geometry and topology with quantum field theory.

In 1994, Simons and his wife, Marilyn, founded the Simons Foundation to support research in mathematics and fundamental sciences. The foundation is the top benefactor of Stony Brook University, Marilyn's alma mater, and is a major contributor to his alma maters, the Massachusetts Institute of Technology and the University of California, Berkeley. Simons was a member of the boards of the Stony Brook Foundation, the MIT Corporation, and the Simons Laufer Mathematical Sciences Institute in Berkeley, and chaired the boards of Math for America, the Simons Foundation, and Renaissance Technologies. In 2023, the Simons Foundation gave $500 million to Stony Brook University, the second-largest donation to a public university in U.S. history. In 2016, the International Astronomical Union named asteroid 6618 Jimsimons, which Clyde Tombaugh discovered in 1936, after Simons in honor of his contributions to mathematics and philanthropy.

James Harris Simons was born on April 25, 1938, to an American Jewish family, the only child of Marcia (née Kantor) and Matthew Simons, and raised in Brookline, Massachusetts.

He received a bachelor's degree in mathematics from the Massachusetts Institute of Technology in 1958 and a PhD in mathematics from the University of California, Berkeley under the supervision of Bertram Kostant in 1961, at age 23. After graduating from MIT, Simons traveled from Boston to Bogotá, Colombia, on a motor scooter.

Simons's mathematical work primarily focused on the geometry and topology of manifolds. His 1962 Berkeley PhD thesis, written under the direction of Bertram Kostant, gave a new proof of Berger's classification of the holonomy groups of Riemannian manifolds. He subsequently began to work with Shiing-Shen Chern on the theory of characteristic classes, eventually discovering the Chern–Simons secondary characteristic classes of 3-manifolds. Later, mathematical physicist Albert Schwarz discovered early topological quantum field theory, an application of the Chern–Simons form. It is also related to the Yang-Mills functional on 4-manifolds, and has had an effect on modern physics. These and other contributions to geometry and topology led to Simons becoming the 1976 recipient of the American Mathematical Society (AMS) Oswald Veblen Prize in Geometry. In 2014, he was elected to the U.S. National Academy of Sciences.

In 1964, Simons worked with the National Security Agency to break codes. Between 1964 and 1968, he was on the research staff of the Communications Research Division of the Institute for Defense Analysis (CRD of IDA) and taught mathematics at the Massachusetts Institute of Technology and Harvard University. Simons also tried starting a trading company named iStar with colleagues including Richard Leibler, but was discovered by management, and the effort failed. After being forced to leave the IDA due to his public opposition to the Vietnam War, he joined the faculty at Stony Brook University. From 1968 to 1978, he chaired Stony Brook's math department. In 1973, IBM asked Simons to attack the block cipher Lucifer, an early but direct precursor to the Data Encryption Standard (DES). In 2004, Simons founded Math for America, a nonprofit organization with a mission to improve mathematics education in U.S. public schools by recruiting more highly qualified teachers.

Simons founded the hedge fund management firm Monemetrics, which he later renamed Renaissance Technologies. He gradually realized that it should be possible to make mathematical models of the data he was collecting. After hiring mathematicians such as Leonard E. Baum and James Ax, Renaissance established the Medallion Fund in 1988.