So unexpected was his win over favourite and 2008 champion Chen Yibing, most fans at the North Greenwich Arena did not even note Zanetti's shock triumph until there was suddenly a loud commotion from a small bunch kitted out in green and yellow.
The 22-year-old gymnast rushed towards his supporters the moment his score of 15.900 flashed up and, had the stands been a little lower, it looked like he would have dived into the crowd.
Instead, he snatched the Brazilian flag thrown his way and ran off on a victory lap having usurped Chen by 0.1 of a point.
Italy's Matteo Morandi won the bronze.
"This medal isn't just for me but for all the athletes and all the gymnasts in Brazil," Zanetti told reporters after he struggled to hold back the tears during the victory ceremony.
China's world and Olympic champion Chen had been expected to retain his crown and seemed to have the gold in his pocket after a solid performance in the strongman event.
He maintained straight bodylines as he lifted himself into a succession of strength moves - such as the inverted cross, the maltese manoeuvre, the handstand - in a routine packed with high difficulty.
When he completed his dismount with a thud, he acted as if the gold was already his, even though all of his remaining challengers had yet to compete.
He held aloft his index finger, kissed it and then let out an almighty roar before running off to kiss the metal frame of the apparatus.
When his score came up, the wagging index finger was on show again, this time accompanied by a cheeky wink into camera.
None of the next six competitors managed to eclipse him until, Zanetti, the final challenger, produced an inspired display to snatch the title away from one of the greatest rings specialists, who was making his final bow on an international stage.
Zanetti picked up Brazil's second gold of the Games, with the first coming in judo last week.
Bulgaria's 39-year-old Jordan Jovtchev, a bronze medallist on the apparatus in 2000 and silver in 2004, bowed out of his sixth and final Olympics in seventh.