William Grant Diering (born 7 May 1986) is a South African swimmer, who specialized in breaststroke events. He finished twelfth in the 200 m breaststroke at the 2008 Summer Olympics, and also set a new South African record (2:06.85) to earn a bronze medal at the FINA World Short Course Championships few months later in Manchester, England. Shortly after the Olympics, Diering edged out Tunisia's Oussama Mellouli by nearly two seconds to claim the 200 m breaststroke title at the African Swimming Championships in Johannesburg with a time of 2:16.00.