The Men's 100 Butterfly event at the 10th FINA World Aquatics Championships swam 25–26 July 2003 in Barcelona, Spain. Preliminary[1] and Semifinal[2] heats swam July 25, with the prelims during the morning session and the semifinals in the evening session. The Final[3] was held on July 26.
At the start of the event, the existing World (WR) and Championship (CR) records were both: