Carlos Tevez's eighth goal of the tournament gave Argentina their first gold medal for 52 years as they beat Paraguay 1-0 in the men's football.
Tevez turned in Mauro Rosales' cross at the near post after 17 minutes.
Argentina, who did not concede a goal in the entire tournament, had already seen a Luiz Gonzalez effort ruled out for offside.
Paraguay had chances but saw both Emilio Martinez and Diego Figueredo sent off in the second half.
Martinez was dismissed after catching Andres Alessandro in the face with his elbow after 67 minutes.
Figueredo saw red eight minutes from time for deliberate handball as he attempted to score.
TOURNAMENT TOP SCORERS
Carlos Tevez (Argentina)
Jose Cardozo (Paraguay)
Tenema Ndiaye (Mali),
Fredy Bareiro (Paraguay),
Alberto Gilardino (Italy)
A crowd of 41,116 braved a scorching Athens morning to watch Argentina end their gold drought.
And they saw new Manchester United signing Gabriel Heinze turn in another solid performance in the Argentine defence.
Sir Alex Ferguson has not been happy that he has not yet seen the £6.9m defender train and will have to carry on waiting as Heinze now goes on World Cup duty.
Boca Juniors striker Tevez, 20, finished the tournament as top scorer.
Argentina had twice finished runner-up in the football - in 1928 and in 1996.