Koji Uehara was born April 3, 1975 was a Japanese professional baseball pitcher who is with the Boston Red Sox. He has played in Nippon Professional Baseball (NPB) with the Yomiuri Giants and for the Baltimore Orioles, Texas Rangers and Red Sox.

A right-handed pitcher, Uehara has a solid career strikeout rate, with 10.4 K/9 and an excellent walk rate at 1.2 BB/9 (until 2013 season). Through the 2013 season, his career 8.74 K/BB is the best in MLB history for a player with at least 100 innings pitched. Uehara won the 2013 ALCS MVP Award, and closed the final game of the 2013 World Series.

 Uehara graduated from the Osaka University of Health and Sport Sciences, and was drafted with the first pick by the Yomiuri Giants in 1998. He had a successful rookie year in 1999, with 15 consecutive wins that broke the all-time rookie record, claimed the Rookie of the Year, Eiji Sawamura Award, and led in wins, ERA, strikeouts and winning percentage.

In 2001 he finished with a 4.02 ERA, the highest of his career. However, in 2002 he rebounded leading the Central League in wins and collected his second Sawamura Award.

He was injured before the 2007 season which made him a late appearance, and in that season, he became a closer instead, recorded an 1.74 ERA with 4 wins, 3 losses and 32 saves. Though showing a good ability both starting and closing, he returned as a starting pitcher in the 2008 season. He left the Giants after that season becoming a free agent and allowing him to play in Major League Baseball.

Uehara is renowned for his performance in international competition. He participated in international events since he was in University, he also participated in Olympic Games twice, as well as the first World Baseball Classic, and participated in Asian Baseball Championships. He has 12 wins and 2 saves, without a loss in his 25 appearances from the above events. He was a member of the Japanese national baseball team which competed in the 2004 Olympic games in Athens. In 2006 he joined Team Japan for the World Baseball Classic and earned 2 wins, improving his unbeaten record in international competition (including amateur appearances) to 12 wins in 21 appearances.

Uehara moved to another team in April 2008. He remained in the 39-out-of-77 men candidate list towards the Beijing Olympics in late June, and was selected to the final 24-men list in mid-July.

Uehara chose not to participate in the World Baseball Classic in 2009. In 1998, Uehara rejected a contract worth $3 million from the then-Anaheim Angels and signed with Yomiuri. The Angels had expressed their continued interest in Uehara, as scouting director Eddie Bane had stated that acquiring either Uehara or Daisuke Matsuzaka was a top priority for the team. However, many other teams, including the Yankees, Dodgers, Red Sox, Mets, and Orioles had shown interest in bidding for Uehara if and when he were to become available.

Uehara asserted his preference in public to be transferred to a Major League Baseball team through the posting system. His efforts had been rebuffed by the Yomiuri Giants front office. He was expected to be eligible for free agency in 2007 (but that was postponed to 2008 due to injury). He became eligible for free agency in April 2008.

On January 13, 2009, Uehara signed a two-year deal with the Baltimore Orioles. He started the 2009 season as the number two starter behind Jeremy Guthrie. Uehara made his big league debut on April 8 against the New York Yankees. On July 30, 2011, Uehara was traded to the Texas Rangers for Chris Davis and Tommy Hunter.  

On December 6, 2012, Uehara agreed to a one-year contract with the Boston Red Sox. Uehara transitioned his role from setup man to closer after season-ending injuries to Andrew Bailey and Joel Hanrahan.

Uehara's 2013 season was one of the most dominant by any relief pitcher in baseball history. His 2013 WHIP of 0.57 in 74.1 innings set the record for a pitcher with 50 or more innings pitched. Between July 9 and September 17, Uehara retired 37 consecutive batters, exceeding the previous franchise record of 32, and nearing Bobby Jenks' MLB record of 41 for consecutive outs by a reliever. Uehara finished the regular season with a 1.09 ERA, a 2.08 xFIP, and struck out 38.1% of batters he faced. He was ranked by Fangraphs as the number one reliever of 2013 in Wins Above Replacement.

Uehara participated in five games of the 2013 ALCS, and was named ALCS Most Valuable Player. In the series he pitched 5.1 innings, allowing 4 hits and no walks; and collected 9 strikeouts. He recorded a save in Game 6 to win the Red Sox their 13th AL pennant.

In Game 4 of the World Series, Uehara picked off St. Louis Cardinals pinch runner Kolten Wong for the last out of a 4–2 Red Sox win. In Game 5, he recorded his seventh save of the postseason, tying the record for most saves in a single postseason. (The next year Greg Holland matched his record for saves in the playoffs, tying John Wetteland, Robb Nen, Troy Percival, and Brad Lidge). Uehara threw the final pitch of the 2013 World Series, closing out a 6–1 win in Game 6.

On July 9, 2014, Uehara was named to his first career All Star Game, replacing injured New York Yankees pitcher Masahiro Tanaka. He struggled near the end of the 2014 regular season and was removed from the closer role on September 5.  He signed a two year extension with the Red Sox on October 30, 2014.

At the start of 2015, Koji signed a two-year deal worth $18 million. Injured in spring training, he came off the DL and on April 14th, shut down the Nationals. He had not successfully closed out a game since August 12th of 2014. He sprinted to the mound, eager to pitch. Three outs later, the Red Sox had their closer back and an 8-7 victory.

But against Detroit, on August 7th, the Sox lost Koji for the rest of the year. The game against the Tigers ended with him being struck on the right wrist and fracturing it, by a line drive.

After closing for the Red Sox since the 2013 season, Uehara was the set-up man in 2016, with the team acquiring closer Craig Kimbrel in the offseason. It was a tough year for him as he posted a 3.45 ERA, his highest as a member of the Red Sox, and wasn’t healthy for a second straight season. He missed the end of July and the entire month of August with a strained pectoral muscle. He returned September 7th, but wasn’t able to pitch back-to-back days or more than an inning in a given game the rest of the season.