The 23-25, 27-25, 25-19, 25-17 victory extends the U.S. team's Olympic winning streak to 11 matches, dating to Beijing.
The fifth-ranked U.S. team dropped the first set but rebounded to win the next two at London's Earls Court, where the Brazilian fans far outnumbered their U.S. counterparts.
On a roll, the team went up 17-10 in the fourth on Russell Holmes' block, even though Brazil had finally inserted star Giba into the match. Reid Priddy spiked for match point.
Priddy finished with 17 points. Leandro Vissotto Neves led No. 1 Brazil with 15.
The Americans went undefeated in Beijing and beat Brazil for the gold. The march came after coach Hugh McCutcheon's father-in-law was stabbed to death at a Beijing tourist site the day before the opening ceremony.
The U.S. men weren't considered among the favourites to medal in London. The team is now led by coach Alan Knipe after McCutcheon moved to the women's team.
But the Americans claimed the silver medal in the recent World League tournament before opening the Olympics with dominant straight-set victories over Serbia and Germany.
Brazil also faced questions coming into the London Games after a disappointing finish in the recent FIVB World League tournament, where it failed to make the semifinal for the first time since 1998.
But the Brazilians have looked strong in London, with sweeps of Tunisia and No. 2 Russia.
Coach Bernardo Rezende had rested star Gilberto Godhoy Filho, known universally as Giba, in the preliminary matches, playing the 35-year-old veteran only sparingly.
Giba, who had surgery on his left leg earlier this year, had both of his knees wrapped and did not start the match. When Brazil fell behind in the fourth set, the Brazilian fans started cheering Giba's name. He entered the match to wild applause.
The U.S. men took their first lead of the opening set when Stanley's ace made it 10-9. Brazil stayed close, but couldn't manage to retake the lead until Vissotto Neves' ace made it 24-23.
David Anderson's ace put the U.S. in front 9-5 to open the second set, and Priddy's kill extended it to 19-13. Brazil scored eight straight points, helped by three straight U.S. errors, to pull ahead 21-19.
After Priddy's ace gave the U.S. a 23-21 lead, his kill looked to claim the set but a questionable call gave the point to Brazil. The United States won it when Sidnei dos Santos Junior's spike went wide.
The United States pulled away 23-18 in the third set when Murilo Endres' kill went long. The team won it when Brazil's blockers couldn't stop Matt Anderson's smash.
The United States and Brazil are in a difficult pool in London, joining Russia, Serbia, Germany and Tunisia. In the other pool are Italy, Poland, Argentina, Bulgaria, Australia and host Britain. The top four teams from each group in the preliminary round advance to the quarterfinals.
In other matches:
— Zbigniew Bartman scored 17 points and Poland defeated Argentina in straight sets to improve to 2-1. Captain Bartosz Kurek added 15 points in the 25-18, 25-20, 25-16 win. Argentina, which fell to 1-2 in pool play. The victory delighted the mostly pro-Team Polska crowd, which sang, chanted and stomped throughout the match. Poland, ranked No. 3 in the world by the sport's governing body, is considered among the favourites in London.
— Russia easily defeated Tunisia in straight sets, 25-21, 25-15 25-23.
— Basketball star Vlade Divac was on hand to cheer Serbia, which fell 3-2 to Germany. Georg Grozer scored a stunning 39 points in the 22-25, 27-29, 25-18, 25-20, 20-18 victory. The Germans and Serbians are both 1-2 in the preliminary round.
— Bulgaria, which upset Poland earlier in the competition, defeated Australia in straight sets. Todor Aleksiev had 16 points. Bulgaria is undefeated in the preliminary round, surprising because the team came to London in disarray. The coach and a top player had resigned to protest their country's leadership of the sport.