Dae-Sung Koo Hangul: 구대성 Hanja: 具台晟 Revised Romanization: Gu Dae-seong McCune-Reischauer: Ku Dae-sŏng Koo is a left-handed pitcher despite being born right-handed. As a child, he suffered an injury which forced him to become left-handed. Koo attended Hanyang University where he was a teammate of Chan Ho Park. Koo played for the South Korean national Olympic team in both the 1992 and 2000 games. He led his team to a bronze medal in 2000 by pitching a 3-1 complete game victory over Japan.

Koo began his professional career in 1993 with the Hanwha Eagles of the Korean Baseball Organization. In 1996, he won both a Gold Glove Award and the MVP of the KBO with a 1.88 ERA, 18 wins, 24 saves and 183 strikeouts in 139 innings pitched.

In 2001, Koo joined the Orix BlueWave of the Japanese Pacific League. A reliever throughout his professional career, he became a starting pitcher with Orix in 2002.

In 2005, the New York Mets signed Koo as a free agent. He pitched well as a reliever during spring training and was selected for the Mets' active roster as the season began. Despite his age, 35, and 12 years experience in Korea and Japan, he is considered a rookie by Major League Baseball.

Koo's most memorable moment as a Major Leaguer occurred on May 21, 2005, when he recorded his first major league hit, a double off of Randy Johnson of the New York Yankees. This came right when Fox TV analyst Tim McCarver commented how Koo's plate appearance was "the biggest give-up at bat". Upon reaching second base, the home crowd and his teammates in the dugout cheered and chanted "KOOOO!" When asked about what he thought about the hit, Koo responded, "Other people say that I swung my bat with my eyes closed. But, of course I saw it -I hit it! It has been 18 years since I batted last - when I was in high school. Occasionally I went to the batting cages and swung the bat a couple of times. But, there really wasn’t any difference." A slow runner with little base running experience, Koo advanced to third base on a sacrifice bunt dropped by the following hitter, José Reyes. However, after Koo slowed down approaching third base, he re-accelerated and ran towards a vacant home plate left uncovered as Reyes was getting thrown out at first base. Yankees' catcher Jorge Posada promptly ran towards home in a foot race with Koo. Upon retrieving the thrown ball from first base, Posada applied the tag onto Koo's left shoulder as he slid head first towards homeplate on a close play. Home plate umpire Chuck Meriwether called Koo safe on the play (despite replays showing that Posada applied the tag on Koo before he touched the plate). With that, the home crowd once again cheered and chanted his last name. His teammates enthusiastically greeted him, with celebrations ranging from his teammates brushing the dirt off his uniform to Doug Mientkiewicz fanning him with a towel.

On March 2, 2006, Koo rejoined the Hanwha Eagles when the Mets sold his contract to them.

Koo is married with two children and resides in Seoul.