William Kiptarus Tanui (born 22 February 1964) is a Kenyan former athlete, winner of 800 m at the 1992 Summer Olympics.

Born in Terik Location of South Nandi District, William Tanui first came to notice relatively late in life, when he won the 1500 m at the Kenyan Commonwealth Games trials in 1989, but ran only sixth at the 1990 Commonwealth Games in Auckland, New Zealand.

In the World Indoor Championships at Seville in 1991, Tanui crossed the line first, but was disqualified for breaking from his lane too early.

In the 800 m final he stormed down the outside, actually finishing in lane 3, to narrowly beat a fellow countryman Nixon Kiprotich for the gold medal.

Tanui was unable to repeat this form in his only appearance at a World Championships, at Stuttgart in 1993, finishing seventh.