"This is a great event," Kennedy said at a reception at the Embassy's residence. "Last fall's victory would not have happened without these two players here tonight."
Earlier in the day, the two pitchers presented Prime Minister Shinzo Abe with a Red Sox jersey and talked baseball with the Japanese leader.
Kennedy, the 56-year-old daughter of late President John F. Kennedy, arrived in Japan in November to become the first woman to serve as U.S. ambassador to Japan.
Tazawa and Uehara played key roles in helping the Red Sox win the 2013 World Series.
The 38-year-old Uehara pitched a perfect ninth inning to finish off a 6-1 victory over the St. Louis Cardinals in Game 6, securing Boston's third championship in 10 years. The 27-year-old Tazawa pitched in 13 of 16 postseason games.
Also attending Tuesday's reception were Japan's first major leaguer, Masanori Murakami, former New York Yankees outfielder and 2009 World Series MVP Hideki Matsui and Japanese conductor Seiji Ozawa, the former music director of the Boston Symphony Orchestra and an ardent Red Sox fan.
Kennedy thanked Major League Baseball for its continuing contributions to the U.S.-Japan relationship.
"You and your players—past and present—are ambassadors of goodwill to millions of fans here in Japan," Kennedy said. "Thank you for continuing the great legacy of baseball diplomacy between our two countries."