Ledecky clocked eight minutes 11.04 seconds to become the only woman – and only swimmer other than the great Michael Phelps – with four Olympic golds in the same event.
It was Ledecky’s ninth Olympic gold, moving her level with former Soviet gymnast Larisa Latynina, and taking her overall tally to 14 medals.
Phelps has the most medals of any Olympian with 28, including 23 golds.
“The four-times record is the one that means the most to me,” Ledecky, 27, said afterwards.
“3 August is the day I won in 2012, and I didn’t want 3 August to be a day I didn’t like moving forwards.
“I put a lot of pressure on myself, so I’m happy I got the job done.”
Earlier on Saturday, Summer McIntosh’s astonishing debut Games continued, with the Canadian 17-year-old securing her third gold with victory in the women’s 200m individual medley.
Athletics
Julien Alfred stormed to the women’s 100m title in Paris to make history as St Lucia’s first Olympic medallist.
As the rain teemed down at a raucous Stade de France, Alfred, 23, dominated the final and sealed victory by a clear margin in a national record 10.72 seconds.
American world champion Sha’Carri Richardson took silver in 10.87, with compatriot Melissa Jefferson (10.92) third.
Jamaican sprint icon Shelly-Ann Fraser-Pryce withdrew from the competition before her semi-final.
Elsewhere, Thea LaFond made history for the Caribbean island nation of Dominica by winning gold in the triple jump to claim the country’s first-ever Olympic medal.
Jamaica’s Shanieka Ricketts took silver and American Jasmine Moore claimed bronze.
American Ryan Crouser became the first man to “three-peat” in the Olympic shot put on Saturday, adding another gold medal to his packed trophy cabinet with a mighty throw of 22.90 metres.
His compatriot Joe Kovacs won his third straight silver and Jamaica’s Rajindra Campbell took bronze.
Gymnastics
Simone Biles won her third Olympic gold of the Paris Games when she took the vault title in emphatic fashion.
After helping the USA to women’s team gold and then taking the all-around title, the 27-year-old reclaimed another of the titles she first won at Rio 2016, having lost them in Tokyo when she pulled out of several events.
She nailed her huge Biles II vault before sticking her ‘easier’ vault – whose difficulty is harder than many attempt as their best vault – to score an average of 15.300.
Brazil’s Rebeca Andrade, the only gymnast who has skills that can come close to Biles and who won the title in Tokyo, took silver with 14.966 and American Jade Carey got bronze with 14.466.
Tennis
Zheng Qinwen became the first Chinese player to win an Olympic tennis singles gold medal when she held off Croatia’s Donna Vekic for a 6-2 6-3 win in a tense final on Saturday.
The 21-year-old sixth seed was given enthusiastic backing by a sizeable Chinese contingent inside Roland Garros’s Court Philippe Chatrier and responded with a composed performance.
Vekic, herself bidding to become Croatia’s first Olympic singles champion, battled hard to try to turn around the match but failed to capitalise on her opportunities.
“This Olympic journey has not been easy, I had a lot of tough fights and some matches I nearly lost,” Zheng, runner-up at the Australian Open this year, told reporters. “But there is a strength holding me. I never give up.
“For the final today the atmosphere was incredible, all the Chinese fans yelling for me. Two years ago that would have been pressure but today I kept calm and handled it well.”