Kim, who allowed the tying run to reach base in the ninth inning of Boston's loss in the opener, was jeered fans at Fenway Park when the public-address announcer read his name.
"I apologize to the fans of the Red Sox, the people of New England and baseball fans throughout the world," the Korean pitcher said in a statement released by the club after the game. "It was an instant, reflexive reaction that I regret... I appreciate the passion our fans have for baseball in Boston; all of us depend on them and their support. I am very sorry."
Kim did not pitch in Boston's 3-1 win in 11 innings that cut Oakland's lead to 2-1 in the best-of-five series. Game 4 is Sunday at Fenway.
The usually reserved Kim raised his right arm and touched the bill of his cap. He then brought the arm down to his right side, raised it again without changing the small smile on his face and put up his middle finger.
Reliever Mike Timlin, the last player introduced before Kim, was upset.
"I was standing right next to him. I was personally embarrassed," said Timlin, who pitched three perfect innings. "He's a man. He's got to control his actions... I was embarrassed for myself and for the Red Sox."
Boston obtained Kim from Arizona for infielder Shea Hillenbrand on May 29. In 44 relief appearances and five starts with the Red Sox this season, Kim was 8-5 with a 3.18 ERA and 16 saves in 19 chances.
Kim gave up game-tying homers for Arizona in Games 4 and 5 of the 2001 World Series at Yankee Stadium.
By Howard Ulman