In Thursday, Smith's national team colleague Kate Douglass set a new record on 200 meters butterfly in Incheon, South Korea, and broke a world record that had stood for 15 years.
Now, Regan Smith, with five OS medals from Paris, including two relay golds, didn't want to be worse.
The American star broke Australian Kaylee McKeown's month-old record by 15 hundredths to 54,41.
McKeown, double Olympic champion from Paris, would have participated in the World Cup, but withdrew to focus on her mental health.
She is outstanding and I would have loved to compete against her. I'm super happy to break the world record, says Smith in Eurovision Sports' broadcast.
Regan Smith won by a wide margin and was 1,73 seconds ahead of second-place Ingrid Wilm, Canada.
Louise Hansson finished fifth in the race with a time of 57,21.
Sara Junevik took the first Swedish podium spot so far in Incheon, the second of three star-studded World Cup events, when she finished second on 50 meters butterfly.
Only Kate Douglass was faster than Junevik and won with a time of 24,73.
Junevik was 54 hundredths behind with a time of 25,27 and shared second place with Lily Price, Australia.
