William Keith Relf (22 March 1943 – 12 May 1976) was an English musician, best known as the lead vocalist and harmonica player for the Yardbirds.

Relf started playing in bands around the summer of 1956 as a singer, guitarist, and harmonica player. His blues harp and vocals were as key to the Yardbirds’ sound and success as were the group’s three superstar lead guitarists Eric Clapton, Jeff Beck, and Jimmy Page.

Relf co-wrote many of the original Yardbirds songs (“Shapes of Things“, “I Ain’t Done Wrong”, “Over Under Sideways Down“, “Happenings Ten Years Time Ago“), later showing a leaning towards acoustic/folk music as the sixties unfolded (“Only the Black Rose”). He also sang an early version of “Dazed and Confused” in live Yardbirds concerts, after hearing musician Jake Holmes perform the song, which was later recorded by the band’s successor group Led Zeppelin. Relf died in the basement of his home in 1976 at age 33 from electrocution while playing an electric guitar. Relf had several health problems throughout his life, including emphysema and asthma. He may have been taking the medications commonly used to treat those diseases at the time, and these may have contributed to his inability to survive the electric shock.