William ("Bill") Ashley Kirby (born September 12 1975 in Perth, Western Australia) was an Australian freestyle and butterfly swimmer of the 1990s and 2000s, who won a gold medal at the 2000 Summer Olympics in Sydney as part of the 4x200m freestyle relay team.

Growing up and being educated at the Christ Church Grammar School, Kirby made his debut at the 1993 Pan Pacific Championships before competing at the 1994 Commonwealth Games in Victoria, Canada and the 1994 World Aquatics Championships in Rome as a butterfly swimmer. After missing selection for the 1996 Summer Olympics in Atlanta, due to glandular fever, he contemplated quitting the sport.

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 which lowered the world record to 7m 8.79s. Training at the Australian Institute of Sport, he gained Olympic selection the following year, where he anchored the Australian team of Thorpe, Klim and Todd Pearson to another world record of 7m 7.05s, leaving the United States more than 5.5s in arrears. Kirby's final international competition was at the 2001 World Aquatics Championships in Fukuoka, Japan, where along with Klim, Hackett and Thorpe, the 4x200m freestyle relay world record was lowered to 7m 4.66s. He also collected a gold for swimming in the heats of the 4x100m freestyle relay, and made the final of the 200m freestyle. He retired after the championships to start a coaching career and business. He now has replaced his gold medal for a cap and thongs, and teaches swimming at Christ Church Grammar School (where he was educated)