The men's 100 metre butterfly event at the 2005 World Aquatics Championships took place between 29 July – 30 July. Both the heats and semifinals were held on 29 July with the heats being held in the morning session and the semifinals being held in the evening session. The final was held on 30 July.

Records

Prior to the competition, the existing world and championship records were as follows.

	Name	Nationality	Time	Location	Date
World record	Ian Crocker Ian Crocker Ian Lowell Crocker is a former American swimmer and five-time Olympic medalist. He is a former world record holder in the 100 meter butterfly ....	United States United States The United States of America is a federal constitutional republic comprising fifty states and a federal district...	50.76	Long Beach Long Beach, California Long Beach is a city situated in Los Angeles County in Southern California, on the Pacific coast of the United States. The city is the 36th-largest city in the nation and the seventh-largest in California. As of 2010, its population was 462,257...	13 July 2004
Championship record	Ian Crocker Ian Crocker Ian Lowell Crocker is a former American swimmer and five-time Olympic medalist. He is a former world record holder in the 100 meter butterfly ....	United States United States The United States of America is a federal constitutional republic comprising fifty states and a federal district...	50.98	Barcelona Barcelona Barcelona is the second largest city in Spain after Madrid, and the capital of Catalonia, with a population of 1,621,537 within its administrative limits on a land area of...	26 July 2003

The following records were established during the competition:

Date	Round	Name	Nationality	Time	WR	CR
July 30	Final	Ian Crocker Ian Crocker Ian Lowell Crocker is a former American swimmer and five-time Olympic medalist. He is a former world record holder in the 100 meter butterfly ....	United States United States The United States of America is a federal constitutional republic comprising fifty states and a federal district...	50.40	WR	CR

Final

Rank	Name	Nationality	Time	Notes
	Ian Crocker Ian Crocker Ian Lowell Crocker is a former American swimmer and five-time Olympic medalist. He is a former world record holder in the 100 meter butterfly ....		50.40	WR
	Michael Phelps Michael Phelps Michael Fred Phelps is an American swimmer who has, overall, won 16 Olympic medals—six gold and two bronze at Athens in 2004, and eight gold at Beijing in 2008, becoming the most successful athlete at both of these Olympic Games editions...		51.65
	Andriy Serdinov Andriy Serdinov Andriy Serdinov is a Ukrainian swimmer, who competed in the 2000 Olympics, 2004 Olympics and the 2008 Olympics. Serdinov also won the bronze medal in the 100 m butterfly at the 2004 Summer Olympics....		52.08
4	Igor Marchenko Igor Marchenko (swimmer) Igor Yuryevich Marchenko is a Russian swimmer who competed at the 2000 and 2004 Olympics.-References:...		52.35
5	Evgeny Korotyshkin Evgeny Korotyshkin Yevgeny Yevgeniyevich Korotyshkin, occasionally spelled "Evgeny", is a Russian swimmer from Moscow.- References :...		52.87
5	Mike Mintenko Mike Mintenko Michael Mintenko is a freestyle and butterfly Olympic swimmer from Canada.-External links:* * -References:...		52.87
7	Kaio Almeida		53.13
8	Ryo Takayasu Ryo Takayasu is a Japanese butterfly swimmer.-Major achievements:2006 Pan Pacific Swimming Championships* 100m butterfly 2nd * 4×100m medley relay 2nd 2007 World Championships* 50m butterfly 10th * 100m butterfly 23rd -Personal bests:...		53.28

Key: WR = World record