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.

Stein received his PhD in 2000, from the University of California, Berkeley, for his dissertation ‘Explicit Approaches to Modular Abelian Varieties’,[1] supervised by Hendrik W. Lenstra.

He is the lead developer of SageMath and founder of CoCalc.

[2] He is considered "a leading expert in the field of computational arithmetic".

[3] Stein was the 2013 recipient[4] of the William Dimick Jenks Memorial Prize Award, presented by SIGSAM, a special interest group of the Association for Computing Machinery.