The 20-year-old clocked eight minutes, 12.68 seconds with China’s Li Bingjie taking silver at 2.78 secs back, a new Asian record, and America’s Leah Smith claiming bronze at 4.54.
Her victory means Ledecky finishes her championships in Budapest with five golds, from the the 400m, 1500m freestyle, 4x100m and 4x200m freestyle, and a 200m freestyle silver.
She has now won 14 titles spanning three world championships.
“It’s a great feeling, it’s always an honour to represent Team USA at this level of a meet,” said Ledecky, the most decorated female swimmer in world championships history.
“I always put my best foot forward and do my best.”
Ledecky suffered the only defeat of her career in a worlds final when she lost over 200m to Italy’s Federica Pellegrini on Wednesday.
“I had some ups and downs this week, but I’m happy to come away with those five golds and a silver,” she said.
“I know I can take a lot away from this week and have learned a lot.”