Florence, 19, topped Australian veteran Joel Parkinson in the final heat at Barra da Tijuca beach.
"This is the best feeling in the world," said Florence, whose early heats were something of a struggle for the tour rookie. "We had a couple of lay days and I had to just sit and think about it. I can't even believe this right now."
Florence relied on his experience surfing similar waves in his native Hawaii to turn in a solid semi-final and then topped Parkinson by a score of 16.37-11.44 in fine 3-to-4 foot surf conditions.
Parkinson's second-place finish was enough to move him into first place in the ASP point standings ahead of fellow Aussies Mick Fanning and Josh Kerr. Fanning and Kerr both made it as far as the semifinals in Rio.
Former points leader Kelly Slater of the United States missed the Billabong Rio due to an ankle injury and slipped to eighth in the point standings.
The ASP tour moves on to Fiji June 3-15.
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