Andrew Triggs Hodge, Pete Reed, Tom James and Alex Gregory led from the start in the Four to extend Britain's dominance over the Olympic event to 16 years.
Britain won their fourth successive title by a quarter of a length from the Australians, with the USA winning the bronze medal.
In the next race, Sophie Hosking and Katherine Copeland - who only came together this year - won gold in the Lightweight Double Sculls.
The British crew were roared to victory by over a length, with China in second and Greece third.
I can't describe what this atmosphere is like. It's beyond words, it's epic, it's magic, it's emotional. The crowd is phenomenal. I'm so excited to be here and so proud.
Tom James
Hodge said afterwards: 'It was just impeccable Rowing. We executed our plan. It was our masterpiece. It took four years to make that.
'Four years, training every day, pulling out everything we had.
'I'm the happiest man in the world. I've been blessed with these guys, I've been blessed with the support of my family and my wife. I'm on cloud nine.'
James said: 'I can't describe what this atmosphere is like. It's beyond words, it's epic, it's magic, it's emotional. The crowd is phenomenal. I'm so excited to be here and so proud.'
I can't believe this is real, that we just won. We just won the Olympics!
Katherine Copeland
Copeland said: 'I can't believe this is real, that we just won. We just won the Olympics! I've been trying all week not to think about it because it's made me cry every time.'
Hosking added: 'It's something we've been working on for so long. I just can't believe it actually happened.'
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