William Arthur Stein (born February 21, 1974, in Santa Barbara, California) is a software developer and previously a professor of mathematics at the University of Washington. He is the lead developer of SageMath and founder of CoCalc. Stein does computational and theoretical research into the problem of computing with modular forms and the Birch and Swinnerton-Dyer conjecture. He is considered "a leading expert in the field of computational arithmetic".