World record holder Ledecky blew away the field in the gruelling, 30-lap race at La Defense Arena, touching the wall in an Olympic record 15.30.02, more than 10 seconds ahead of France's silver medallist Anastasiia Kirpichnikova.
Germany's Isabel Gose took bronze.
Ledecky now shares the record for most Olympic gold medals in women's swimming with American Jenny Thompson, who won eight relay titles over three Olympics from 1992-2000.
Her collection of Olympic medals now equals the all-time record of 12 in women's swimming shared by Thompson, Americans Dara Torres and Natalie Coughlin, and Australian Emma McKeon.
One more gold medal in Paris will bring her haul from four Games to nine, matching the current mark set by gymnast Larisa Latynina.
With the 800m free and possible relays still to come, Ledecky could leave Paris officially crowned the greatest ever in her sport.
