Sebastian Eduardo Crismanich won the men's -80kg taekwondo category for Argentina's first gold of London 2012.
Crismanich won 1-0 in a tense final
against Nicolas Garcia Hemme of Spain.
The 25-year-old sparked exuberant celebrations after registering the only scoring kick with 22 seconds remaining.
Lutalo Muhammad had earlier become the first British man to win an Olympic medal in the sport by taking one of the two bronzes;
the other went to Italy's Mauro Sarmiento.
Crismanich was a surprise winner, with favourites Ramin Azizov, Steven Lopez and Yousef Karami all going out early, and he let the tears flow during the medal ceremony.
The Argentinian said: "I can't express with words what I feel and what is happening to me. It's a dream.
"I have to say thank you to my family and to my colleagues. And to all the people of Argentina, I would like to give them a piece of this medal."