She screamed and slapped the water when she looked at the scoreboard to show her time of 55.03 seconds, taking 0.20 seconds of the mark set by Japan's Shiho Sakai in Berlin in 2009.
"I thought a lot last night about how I did yesterday," Hosszu told reporters. "Coming into today, I was just trying to do my best.
"During the World Cup season, I was three times close to the world record, about 14 hundredths of a second, so I'm just really excited that I finally got (it)."
South African Olympic champion Chad le Clos continued the world record-breaking run at the five-day championships when he claimed the men's 100 butterfly, adding the title to his surprise 200 freestyle crown.
The 22-year-old, who upset Michael Phelps to snatch the Olympic 200 butterfly title in London in 2012, clocked 48.44 seconds, taking 0.04 seconds off the mark set by Russia's Evgeny Korotyshkin in 2009.
"Timing is important to get the world record," le Close said. "You have to race to win.
"There was lots of pressure from the media and others on my shoulders and I wanted to show ...the world that I am the best swimmer in the world."
Belmonte, who beat Hosszu in the 200 butterfly and 400 individual medley in world record time, clinched her third title of the championships when she won the 800 freestyle ahead of Britain's Jaz Carlin.
Brazil's Felipe Franca Silva claimed three gold medals on Thursday when he won the men's 100 breaststroke by producing a championship record swim to pip fastest qualifier Adam Peaty of Britain with world record holder Cameron van der Burgh fourth.
Franca Silva also helped Brazil to the men's 4x50 medley relay in a world record time and the 4x50 mixed medley team to gold. Both are new events at the championships.
(Reporting by Greg Stutchbury in Wellington; Editing by Ed Osmond)
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