In the second semifinal heat, Meilutyte touched in 29.48 seconds, beating the record set in the morning preliminaries by Russia's Yuliya Efimova.
Efimova was swimming the same heat as the 16-year-old and touched second in 29.88, just off the mark of 29.78 she held for not even a day. She had broken the previous record set by American Jessica Hardy in 2009 at the height of the rubberized suit era, 29.80.
The 50 breaststroke is not an Olympic event.
It was the second world record for Meilutyte at these championships. She also set one in the semifinals of the 100 breast.