The competition started with the 100 in heavy rain on Thursday morning.
Andrei Krauchanka of Belarus took silver at 8,551, and Leonel Suarez of Cuba was third in 8,527.
Roman Sebrle of the Czech Republic, the reigning world and Olympic champion, finished sixth.
Dan O'Brien was the last U.S. athlete to win the Olympic decathlon, at Atlanta in 1996.
