William Ashley Kirby OAM (born 12 September 1975) is an Australian swimmer who was competitive on an international level in the nineties and early 2000s.

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 4 × 200 m freestyle relay team (along with Michael Klim, Ian Thorpe and Grant Hackett) that lowered the world record, at the time, to 7m 08.79s.

That record held firm for nearly six years until United States relay team clocked in at 7m 03.24s at the 2007 World Championships in Melbourne.