|Gold medalist Katie Ledecky of the U.S. poses with her medal at the women's 1500m freestyle victory ceremony during the World Swimming Championships at the Sant Jordi arena in Barcelona July 30, 2013. REUTERS/Michael Dalder|
The 16-year-old from Bethesda, who also won the 400 freestyle gold medal on Sunday, powered away from defending champion Lotte Friis of Denmark in the final stages to break the previous best of 15:42.54 set by compatriot Kate Ziegler in 2007.
It was the second world record of the championships in the specially constructed pool at the hilltop Palau Sant Jordi after Ruta Meilutyte's mark in qualifying for the 100 metres breaststroke on Monday.
Lithuanian Meilutyte, born two days after Ledecky in March 1997, went on to claim a first world championship gold for the tiny Baltic nation when she triumphed in Tuesday's final.
She was unable to better Monday's effort but still produced the second-fastest time ever of 1:04.42, just seven hundredths slower than her record 1:04.35.
Ledecky's team mate Missy Franklin, who won four gold medals and one bronze in last year's London Olympics, secured her second gold of the week with victory in the 100 metres breaststroke and Matt Grevers claimed the 100 metres backstroke title.
Ryan Lochte, the second-most decorated men's world championship medallist after compatriot Michael Phelps, provided a rare moment of disappointment for the U.S. team when he could manage only fourth in the 200 metres freestyle final.
Frenchman Yannick Agnel followed up on his triumph at last year's London Olympics to take gold and Conor Dwyer salvaged some pride for the U.S. with silver.
"We've had an absolutely incredible evening," an excited Franklin, who is swimming eight events in Barcelona, told reporters.
Ledecky sat on the shoulder of Friis for much of their two-horse race but the Dane was left trailing by a powerful late charge from the Olympic 800 metres champion and finished with silver in 15:38.88, also inside Ziegler's mark.
Lauren Boyle of New Zealand was third in 15:44.71 ahead of Spain's Mireia Belmonte Garcia in 15:58.83.
"It was really tough, my hardest race ever," Ledecky told reporters.
"I knew we were going pretty fast and I figured that whoever came out on top was probably going to get the world record.
"I had to be careful not to push it too early or push it too late and just touch the wall first. Around the last 200 I knew I could take off."
Succeeding Ziegler as the fastest woman over 1,500 metres was extra special, Ledecky said, as the 2011 world championship silver medallist behind Friis hails from the same part of the United States.
"It means the world to me," said a still-dripping Ledecky, goggle marks visible around her eyes.
"Kate Ziegler is from my area and I have looked up to her my whole life.
"I am really honoured to break that world record and to keep it in the Potomac valley.
"I'm still 16 so I hope I have plenty years left of swimming and I guess we'll see where my limit is."
Franklin had a gruelling day as she had to qualify for Wednesday's 200 metres freestyle final around an hour after claiming backstroke gold.
It was the 18-year-old's fifth world championship title, already putting her fourth among her compatriots.
"It was tough but it was awesome," she told reporters.
"It was nice to have the final before the semi-final - it was the other way around in London," she added.
"Hopefully I will keep the momentum going, hopefully Team USA will too."
Agnel added a world 200 metres freestyle title to his Olympic gold when he comfortably outgunned Dwyer, with Russia's Danila Izotov taking bronze ahead of Lochte.
"This medal means a lot," Agnel, who has recently started training with Phelps's former coach Bob Bowman, told reporters.
"I just wanted to enjoy myself, I was relaxed," he added.
"Even if a world title is less important than an Olympic title this is hugely significant."
Lochte, 28, who is swimming seven events this week, said he had made crucial mistakes.
"I am a bit disappointed with the time but you know I've got to keep moving forward," he said on Spanish television.
"There were a lot of places where I messed up but hopefully I can fix it in the next three years (before the 2016 Olympics) and I can make it better."
There was consolation for Lochte when team mate Matt Grevers took gold in the backstroke and compatriot David Plummer claimed silver ahead of France's Jeremy Stravius.
Stravius tied for gold with compatriot Camille Lacourt at the last world championships in Shanghai two years ago and Lacourt finished fifth this time around behind Ryosuke Irie of Japan in fourth.
Four finals are scheduled for Wednesday; the men's 50 metres breaststroke, 200 metres butterfly and 800 metres freestyle and the women's 200 metres freestyle.
(Additional reporting by Emma Pinedo in Madrid; Editing by John Mehaffey)