Phelps joined Conor Dwyer, Ryan Lochte and Townley Haas to hit the wall in 7 minutes 0.66 seconds at the Olympic Aquatics Stadium on Tuesday, reports Xinhua.
Britain's Stephen Milne, Duncan Scott, Dan Wallace and James Guy took silver, 2.47sec behind the US, and Japan's Kosuke Hagino, Naito Ehara, Yuki Kobori and Takeshi Matsuda pocketed bronze, 2.84sec further back.
Phelps, who the 200m butterfly final less than an hour earlier, swum the final leg of the race to loud cheers from the packed arena. It was the American's 21st Olympic gold medal.
Phelps has reduced his competition schedule in Rio as he focuses on butterfly and medley events.
He is aiming to become only the third athlete to win gold in the same event at four consecutive Olympics, joining Al Oerter (discus) and Carl Lewis (long jump).
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