Swimming at the 2016 Summer Olympics – Women's 800 metre freestyle

The women's 800 metre freestyle event at the 2016 Summer Olympics took place between 11–12 August at the Olympic Aquatics Stadium.

U.S. swimmer Katie Ledecky smashed a new world record to defend her Olympic title in this event and to successfully complete a distance freestyle treble at a single edition for the first time, since Debbie Meyer did so in 1968. Dominating the race from the start, Ledecky quickly dropped two seconds under a world-record pace, as she pulled further away from the field to overturn her own existing standard with a gold-medal time in 8:04.79. Separated the leader by 11.38 seconds, Great Britain's Jazmin Carlin edged out the Hungarian challenger Boglárka Kapás at the final lap for her second silver of the meet in 8:16.17. Meanwhile, Kapás faded down the stretch to earn a bronze in 8:16.37, two tenths of a second short of Carlin's time.

London 2012 runner-up Mireia Belmonte slipped off the podium to fourth in a Spanish record of 8:18.55. Outside the 8:20 club, Australia's Jessica Ashwood (8:20.32) and Ledecky's teammate Leah Smith (8:20.95), bronze medalist in the 400 m freestyle, picked up the fifth and sixth spots respectively, finishing 63-hundredths of a second apart from each other. Denmark's Lotte Friis (8:24.50) and Germany's Sarah Köhler (8:27.75) rounded out the championship field.

Ledecky also threw down the existing Olympic record in 8:12.86 to top the field of twenty-seven swimmers in the prelims, slashing 1.14 seconds off the standard set by Great Britain's Rebecca Adlington on a since-banned high-tech bodysuit in Beijing eight years earlier.