The Men's 100 Backstroke event at the 10th FINA World Aquatics Championships swam 21 – 22 July 2003 in Barcelona, Spain.

Preliminary[1] and Semifinal[2] heats swam on July 21; the Final[3] was held on July 22.

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