Ledecky, 15, led from start to finish as the expected duel between Britain's Rebecca Adlington and Denmark's Lotte Friis failed to materialise.
Instead, Ledecky went straight out and moved further ahead from the halfway stage, under world record pace all the way until the final metres to touch in 8:14.63, with only Adlington ever having gone faster when she set a world record in Beijing four years ago.
With 200m to go, Spaniard Mireia Belmonte Garcia attacked to overhaul Adlington and finish second in 8:18.76.
Adlington picked up her second third place of the week, following her 400m bronze, touching in 8:20.32.
Meanwhile, Florent Manaudou claimed the final medal of the night in the pool, winning the men's 50m Freestyle.