Sjöström on Sunday set a new world record in the 100 metres butterfly semi-final at the World Championships in Kazan, Russia, with a time of 55.74 seconds.
The 21-year-old smashed the previous record set by American Dana Vollmer who swam with a time of 55.98 seconds in the London Olympics.
“It was in my plan to set a record in the semi-final, because in the final I am in a different physical state,” said the defending world champion.
“I was quite slow in the first 50 metres, but later I sped up. And I had been waiting to set this record for so long.”
The champion had hinted at the record one month ago at this year's Swedish Championships.
“I'm waiting for the world record for the World Cup. I know I have it in me,” Sjöström said after swimming with a time of 56.96 seconds.
Whether Sjöström can keep up her performance for the final on Monday remains to be seen.
“I’ll see if I have more to give in the final,” she was quoted as saying by Dagens Nyheter. “I’ll look at this race and then we will see.”
This isn’t the first world record for Sjöström who last year broke the world record for 50 metres butterfly at the Swedish Championships.
Meanwhile, on Sunday China's Sun, the reigning 400, 800 and 1500m world champion and record holder over the mile, clocked three minutes 42.58 seconds to win his final by more than a second as Britain's James Guy took silver.
And American teenager Ledecky, a triple world-record holder, built a commanding lead in her final and came within a second of her own world record in the event, but had to make do with a winning time of 3:59.13 mins – a new championship record.
“This gets the momentum going and I couldn't be happier,” beamed the 18-year-old, who has only just graduated from high school.
A gold medal dream came true for sisters Cate and Bronte Campbell as they helped Australia win the women's 4x100m freestyle title.
The Queensland siblings paired up as Cate, the world 100m freestyle champion, anchored the team to set a new championship record of three minutes 31.48 seconds.
The Netherlands won silver at 2.19 secs back while the United States, including six-time world gold medallist Missy Franklin, won bronze at 3.13 back.
After the US team failed to qualify from the morning's heats for the men's 4x100m freestyle relay for the first time in world championships history, the French quartet retained their title.
Mehdy Metella, Florent Manaudou, Fabien Gilot and Jeremy Stravius clocked three minutes, 10.74 seconds as Russia finished just 0.45secs behind to claim silver with loud encouragement from the home crowd in Kazan.
Italy took bronze at 1.79secs back.
There will be a battle royal in the men's 100m breaststroke final on Monday as world-record holder Adam Peaty goes head-to-head with Olympic champion Cameron van der Burgh.
The pair took turns breaking the championship record in the morning heats, then the evening's semi-finals.
After Peaty broke it in the morning heats, South Africa's Van der Burgh won his semi-final in a new championships record of 58.49secs, only for the 20-year-old Briton to break it again in his semi by clocking 58.18secs.
“It is very good for the sport, very good for breaststroke to have two people going at each other,” said Peaty, who is making his worlds debut.
“I am just going out there and doing it, there is no pressure, nothing.”