The quartet of Allison Schmitt, Leah Smith, Maya Dirado and Ledecky won in a time of seven minutes 43.03 seconds, with Australia second and Canada third.
America trailed Australia but Ledecky, the 200m and 400m freestyle champion, overtook Tamsin Cook on the last leg.
Ledecky, 19, has now won four Olympic gold medals during her career.
Source:
BBC