|Katie Ledecky of the U.S. reacts after winning and setting a new world record in the women's 800m freestyle final during the World Swimming Championships at the Sant Jordi arena in Barcelona August 3, 2013. REUTERS/Michael Dalder|
The 16-year-old, who smashed the 1,500 metres world best to win gold on Tuesday and also won Sunday's 400 metres, clocked a time of eight minutes 13.86 seconds at the Palau Sant Jordi in Barcelona.
She was inside the previous best of 8:14.10 set by Rebecca Adlington to win gold at the Beijing Olympics in 2008.
It was her fourth title of the week - she also swam in the winning U.S. 4x200 freestyle relay team - and she is only the second woman to win all three distance freestyle titles at a single world championships.
German Hannah Stockbauer won the treble at the 2003 edition, also in Barcelona.
Dane Lotte Friis, who finished second to Ledecky in the 1,500, took 800 silver in 8:16.32 and Lauren Boyle of New Zealand, also third in the 400 and 1,500, won the bronze in a time of 8:18.58.
(Reporting by Iain Rogers, editing by Ken Ferris)