The US team of Ryan Murphy, Cody Miller, Phelps and Nathan Adrian clocked 3:27.95 to finish 1.29 seconds ahead of Britain's Chris Walker-Hebborn, Adam Peaty, James Guy and Duncan Scott on Saturday, reports Xinhua.
Australia, represented by Mitchell Larkin, Jake Packard, David Morgan and Kyle Chalmers, took bronze in 3:29.93.
Phelps began the butterfly leg with the USA in second position, but he set a blistering pace to give his team the lead by the end of his 100m.
The victory gave Phelps his 23rd Olympic gold medal. The 31-year-old extended his record as the most successful Olympian in history with six medals - five gold and one silver -- at the Rio Games.
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