The Men's 100 Breaststroke event at the 10th FINA World Aquatics Championships swam on 20–21 July 2003 in Barcelona, Spain.

Preliminary[1] and Semifinal[2] heats swam on 20 July, with the Final[3] on 21 July.

At the start of the event, the existing World (WR) and Championship (CR) records were both: