William Lyell (born August 10, 1984) is an American former professional boxer who competed from 2003 to 2013. He challenged for the IBF middleweight title in 2010.
In an interview with Peter Czymbor, Lyell stated that he had an amateur record of "about 35-20" in "about 50-55 amateur bouts".
In February 2009, Lyell upset an undefeated John Duddy in a ten-round fight by split decision.
He then lost a very tough fight to Sebastian Sylvester.
On June 5, 2010, Billy lost to undefeated champion Julio César Chávez, Jr. The bout was held at the Estadio Banorte in Culiacán, Mexico.