William ("Bill") Ashley Kirby (born 12 September 1975 in Perth, Western Australia) is an Australian swimmer who was competitive on an international level in the nineties and early 2000s. He specialized in freestyle and butterfly and won a gold medal at the 2000 Summer Olympics in Sydney as part of the 4x200m freestyle relay team.
Kirby found his way back into the national team at the 1997 FINA World Short Course Swimming Championships in Gothenburg, Sweden. Although he missed selection for the 1998 World Aquatics Championships in his home town, he bounced back to qualify for the 1998 Commonwealth Games in Kuala Lumpur, where he won a silver medal in the 200m butterfly. At the 1999 Pan Pacific Championships in Sydney, he was a member of the 4x200m freestyle relay team (along with Michael Klim, Ian Thorpe and Grant Hackett) that lowered the world record, at the time, to 7m 8.79s. Training at the Australian Institute of Sport, he earned an Olympic berth the following year, anchoring the Australian 4x200m relay team of Thorpe, Klim and Todd Pearson to another world record of 7m 7.05s.
Kirby's final international competition was at the 2001 World Aquatics Championships in Fukuoka, Japan, where along with Klim, Hackett and Thorpe, he lowered the 4x200m freestyle relay world record to 7m 4.66s. That record held firm for nearly six years until United States relay team clocked in at 7m 3.24s at the 2007 World Championships in Melbourne, Australia.
Kirby also collected a gold for swimming in the non-final heats of the 4x100m freestyle relay and also swam in the finals of the 200m freestyle individual event. He retired after the championships to start a coaching career and business.
He now teaches swimming at Christ Church Grammar School, the school at which he was educated.