Britain, the world champions, led throughout and finished well clear of Germany, who took silver, and the Netherlands, who claimed bronze.
It is Britain's third gold medal of the rowing competition, and follows the women's eight earlier winning silver.
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Team GB have now won eight golds in Rio and lie third in the overall medal table, one ahead of Japan.
Great Britain finish top of the rowing medals table with five medals: two silvers and three gold - one gold medal fewer than at London 2012.
Andrew Triggs Hodge and Pete Reed are now three-time Olympic gold medallists having won titles in Beijing and London in the men's four.
"I can't put it into words. We've never been so ready. They're an amazing bunch of guys, that was a big, big, race," said Reed.
Scott Durant, Tom Ransley, Matt Gotrel, Paul Bennett, Matt Langridge, Will Satch and cox Phelan Hill completed the victorious British line-up.
Coach Jurgen Grobler said: "There are no secrets. Hard, hard, work, having vision of what is coming up and running a good training programme - and they're still having fun."
Sir Steve Redgrave, five-time Olympic rowing champion
"I think they did the plan perfectly. The surprise I did have was that the Germans didn't go with them.
"GB went out in front and led the field. They struggled in the last 250 metres but the job was done. There wasn't one doubt through the full race.
"We have topped the Olympic rowing table and that is very, very impressive."
James Cracknell, double Olympic champion
"That was brilliant. GB delivered exactly what they have trained for. They were not fazed, they dug deep and stuck to the plan. I am so chuffed for them. What a race!"
Greg Searle, former Olympic champion on Twitter
"Massive congratulations to both GB Rowing eights. Cool, calm and brave from everyone. Well done everyone involved".
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Great Britain's men's eight won gold in Rio to reclaim the Olympic title for the first time since 2000.