The team will play for the title Saturday against the winner of a later semifinal between Brazil and Japan.
"It's great to be in this position," U.S. middle blocker Christa Harmotto said. "It's a position we've worked for for four years, and we're exactly where we want to be."
The top-ranked United States has dropped just two sets in London. In the latest victory, Destinee Hooker scored 24 points.
The American women made it to the final at the 2008 Beijing Games but settled for the silver medal, falling 3-1 to Brazil. The team has won silver twice and bronze once since volleyball joined the Olympics in 1964. They've yet to win a gold.
The U.S. got an emotional boost for the match with the return of captain Lindsey Berg, who was held out of the team's quarterfinal victory over the Dominican Republic with an injury to her lower left leg.
"It's game time and I feel great, and I don't care how I feel after Saturday," Berg said of the final, when the Americans will face either Brazil or Japan for gold.
Fifteenth-ranked South Korea upset No. 4 Italy in four sets Tuesday to advance before losing to the Americans. South Korea's best result in Olympic play came at the 1976 Montreal Games.
The United States has a 6-2 record against South Korea in Olympic matches, including a 3-1 U.S. victory in the opening match of the tournament.
The semifinal was tight at the start, but the United States pulled ahead 20-16 in the first set on Hooker's kill. The South Koreans denied the U.S. its first chance at set point before Kim Yeon-koung's serve sailed out to give it to the Americans for set point.
Jordan Larson's spike made it 15-10 in the third set, but South Korea evened it at 18 on Kim's ace. The U.S. wouldn't let the South Koreans take the lead. Hooker's monster spike set up Logan Tom's kill for match point as the crowd at Earls Court chanted "U-S-A! U-S-A!"
Berg, a three-time Olympian, hurt her leg in the Americans' final preliminary-round match against Turkey on Sunday, and the U.S. kept quiet about when she might return
Berg warmed up before the U.S. women's volleyball straight-set victory the Dominican Republic in the quarterfinals on Tuesday night but didn't play. Courtney Thompson started in her place.
After the match, U.S. coach Hugh McCutcheon made a point of embracing Berg. It will be the second straight Olympic final for McCutcheon, who guided the American men to a gold medal in 2008.
"That's great that she feels so positive about it," McCutcheon said. "I had time to give her a hug and tell her nice job. If she feels good then the rest of us do as well."
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