The American great was untouchable, coming home in 15mins 30.02secs, an Olympic record and more than 10 seconds ahead of France's Anastasiia Kirpichnikova in second. Germany's Isabel Gose won bronze.
Ledecky went out hard and was a body length clear after 100m with her lead growing to five seconds at the halfway mark.
She never relented, her advantage continuing to build as she reinforced her reputation as the greatest distance swimmer the sport has seen.
The result was never in doubt with the 27-year-old setting the last six world records and now holding the fastest 20 times ever swum.
It was her eighth Olympic gold, and 12th medal overall after adding a bronze in the 400m freestyle behind Australian winner Ariarne Titmus.
Ledecky will also swim the 800m in Paris, with Titmus standing in her way of winning a fourth straight title.
