The American had previously won 200 and 800m gold at the championships at the Gold Coast Aquatic Centre in Queensland, and completed a treble of impressive performances on Saturday.
Ledecky remained under world-record pace throughout much of the race, eventually touching the wall having shaved 0.49secs off her previous best time.
"It's a great feeling, it never really gets old," Ledecky said of her records.
"I have some long-term goals. We're not quite there yet, but we'll get there.
"I knew I could be right where I was at nationals, or slightly better. I'm really pleased with that."
Elsewhere, Michael Phelps claimed his second gold of the games, beating rival Ryan Lochte in the 100m butterfly.
After victory in the 4x200m freestyle relay on Friday, Phelps reached finished first in a time of 51.29secs, 0.38secs ahead of Lochte. Phelps is now guaranteed a place on the American team for the world championships next year.
"It definitely feels good to have that sort of confidence back to be able to do it in an individual event," said Phelps.
"What it does is guarantee me a spot on the world championship team next summer which is the biggest and most important thing.
"I know there is a lot can happen in a year with training, I'm looking forward to it, it's a building year for me."