William Nelson Joy (born November 8, 1954) is an American computer engineer and venture capitalist. He co-founded Sun Microsystems in 1982 along with Scott McNealy, Vinod Khosla, and Andy Bechtolsheim, and served as Chief Scientist and CTO at the company until 2003.

He played an integral role in the early development of BSD UNIX while being a graduate student at Berkeley, and he is the original author of the vi text editor. He also wrote the 2000 essay "Why The Future Doesn't Need Us", in which he expressed deep concerns over the development of modern technologies.

Joy was elected a member of the National Academy of Engineering (1999) for contributions to operating systems and networking software.