The 21-year-old American star touched the wall in 15min 20.48sec at the Pro Swim event in Indianapolis, obliterating her previous best of 15:25.48 set at the World Championships in Kazan in 2015.
"I was pretty surprised when I saw the '20,'" Ledecky said with a little laugh, adding that she thought as the swim unfolded that she would be somewhere under 15:30, maybe. "When I saw the 15:20 I was pretty shocked," she said.
This week's meet in Indianapolis is Ledecky's first since turning professional following the NCAA collegiate championships in March.
"My first pro swim -- that's one I'll never forget," said Ledecky, who added that so far she hadn't felt any different in her first meet as a professional. Ledecky won gold in the 800m at the 2012 Olympics and followed it up with four golds and a silver at the Rio Olympics two years ago.
The 1,500m free will be introduced as an Olympic event for women at the 2020 Olympics in Tokyo. Ledecky was under world-record pace for much of Wednesday's race and touched 49 seconds clear of the nearest finisher in her heat. She now owns the eight fastest times in history in the event dating to 2013 and has 14 career world-record swims.
"It's a feeling that never gets old," said Ledecky, who hadn't set a world record since the Rio Games.
"I knew I was going to have a good swim," Ledecky said. "I've just been training really, really well, doing some things that I haven't done before.
"I didn't know if the good training that I've put in these past six weeks was going to translate immediately here or if it was going to be down the road. I got into the race and felt good and just tried to hold steady the whole way."