Walsh crushed Therese Alshammar's 15-year-old world record in the trials with 36 hundredths to 24.02.
In the semifinal at the Duna Arena in Budapest, she struck again and went under 24 seconds, 23.94.
Breaking a half second off the old world record is completely crazy, says Walsh, who has earned $50,000, 550,000 kronor, in world record bonuses.
Sara Junevik missed the final by a single hundredth and finished tenth on 25.14.
The world records stood like poles in the ground during the first day. During the final's first four events, a world record was broken every time.
The 18-year-old Canadian star Summer McIntosh, three-time OS gold medalist in Paris, won the final on 400 meters freestyle in 3.50,25.
It was a breaking of Chinese Li Bingjie's previous record by a whole 1.05.
Swiss Noe Ponti polished his own world record on 50 meters butterfly with seven hundredths to 21.43.
American Kate Douglass broke Hungarian Katinka Hosszu's ten-year-old world record on 200 meters medley with 22 hundredths to 2.01,63.
The final is ongoing.
