The Men's 100 metre butterfly competition at the 2019 World Championships was held on 26 and 27 July 2019.

[1][2] The defending champion Caeleb Dressel broke Michael Phelps' ten-year-old record by 0.32 second in the semi-final, setting a new world-record time of 49.50 seconds.

[3] Dressel won the final held the following day, defending his title.

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

The following new records were set during this competition.