Destinee Hooker had 23 points and Jordan Larson added 18 for the top-ranked Americans, who won 25-18, 25-17, 22-25, 25-21 to improve to 2-0 in pool play at Earls Court.
Sheilla Castro had 15 points for No. 2 Brazil, which was energized after a third-set victory but lost on Logan Tom's floater in the fourth set.
Brazil has lost five straight international matches to the U.S. The teams are 21-21 in career play.
Larson, a wing spiker making her Olympic debut, hit a floater to put the Americans up 23-17 in the first set. She spiked for set point, as Brazil looked out of sorts.
The U.S. team pulled away in the second set with a 7-point run, capped by Tom's spike. Brazil twice held off the U.S. at set point and looked to have some momentum, until Hooker's kill ended it.
Brazil rebounded by going up 8-3 early in the third, but the Americans rallied for a 16-15 lead on Christa Harmotto's block. It was back and fourth until Harmotto's attempt to return Thaisa Menezes' spike went awry.
The U.S. opened the Olympics with a 3-1 victory over South Korea. The Brazilians defeated Turkey in five sets in their first match.
The Americans took the top spot in the world rankings at the end of 2011, ending Brazil's four-year run at No. 1.
China, Serbia, Turkey and South Korea are in the six-team pool with the U.S. and Brazil. Japan, Russia, Italy, Algeria, the Dominican Republic and Britain make up the other pool. The top four teams from each group advance to the quarterfinals.
The U.S. has been playing well under coach Hugh McCutcheon, who coached the U.S. men to the gold medal in Beijing before shifting to the women's team. The American women have never won an Olympic gold medal in volleyball. The sport joined the Olympics in 1964.
In earlier matches, China moved to 2-0 in its group after defeating Turkey, 3-1 (25-20, 25-20, 29-31, 25-22). Kim Yeon-Koung had a staggering 34 points to lead South Korea to a 3-1 victory over Serbia. South Korea is 1-1 in its pool. Russia went up 2-0 in pool play after downing the Dominican Republic 3-1 (25-23, 25-15, 24-26, 25-22).