Felix Sanchez maintained his dominance of the 400m hurdles to add the Olympic title to his world crown in Athens.
The Dominican Republic athlete, who has not lost since July 2001, surged away from the field in the home straight to win in a season's best 47.63 seconds.
American James Carter was level with Sanchez with two hurdles left but tied up to finish out of the medals.
Jamaican Danny McFarlane took the silver in 48.11 with Frenchman Naman Keita claiming the bronze in 48.58.
The New York-born Sanchez extended his unbeaten streak to 43 races as he claimed only his country's second Olympic medal, and its first gold.
"You'd have thought after 42 races it would get easier," he said.
"The confidence was still there, but pressure wise I was very emotional coming into the stadium after all the hours of waiting with the day off.
"Going into the blocks it was all or nothing."
Slower out of the blocks than usual after being credited with a false start, the 26-year-old was under pressure from Carter as they approached the home straight.
But as the American went backwards, Sanchez powered to victory to make up for his disappointment at failing to reach the final in 2000.
He has worn a flashing red wristband on his right wrist - a souvenir from the closing ceremony in Sydney - at every race since as a reminder of that setback.
But he can now discard it after ending the United States' dominance of the event across five Olympics from 1984 to 2000.
Ironically, Sanchez switched his allegiance to the country of his parents' birth after finishing sixth in the US trials for the 1999 World Championships.
"It's a matter of pride, feeling that there are many Dominicans following me," he added.
"The Dominican Republic is a third world country and sport is very much at its heart.
"When I talked to my father after the race he said it is going crazy there and I look forward to going back there."
Sanchez is still some way short of the 122-race unbeaten streak over nine years established by two-time Olympic 400m hurdles champion Ed Moses.
McFarlane took silver having finished eighth in the 400m flat final in Sydney.