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

Born in Kemeloi, Kenya , 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 same year he won the 800 m at the African Championships in Cairo. In the World Indoor Championships at Seville in 1991, Tanui crossed the line first, but was disqualified for breaking from his lane too early.

Tanui's greatest moment came at the 1992 Olympic Games in Barcelona. 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.

Tanui concentrated more on the 1500 m in the latter part of his career, and he finished fifth at that distance in the 1996 Olympic Games in Atlanta. At the World Indoor Championships, Tanui finished third in the 1500 m at Paris in 1997, and fourth at Maebashi in 1999.