The US team of Allison Schmitt, Leah Smith, Maya Dirado and Ledecky finished in 7 minutes 43.03 seconds at the Olympic Aquatics Stadium on Wednesday evening, reports Xinhua.
Australia's Leah Neale, Emma McKeon, Bronte Barratt and Tamsin Cook took the silver, 1.84 seconds behind.
Canada's Katerine Savard, Taylor Ruck, Brittany Maclean and Penny Oleksiak grabbed the bronze, 2.36 seconds further back.
Ledecky began half a body length behind Australia's Cook but reeled in her rival after the first 50m and never looked back.
It was the 19-year-old's third gold medal of the Rio Games. She also won the 200m freestyle and 400m freestyle finals in addition to the silver medal in the 4x100m freestyle.
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