The Mets signed Dae-Sung Koo as a free agent prior to the 2005 season. A native of South Korea, Koo pitched professionally in South Korea as well as in Japan. He also helped lead South Korea to a Bronze Medal in the 2000 Summer Olympics
Koo made his Major League debut on Opening Day in 2005 and pitched a scoreless inning of relief against the Cincinnati Reds. The Mets lost 7-6. Working out of the bullpen, Koo pitched effectively as a left-handed specialist for the Mets. He posted a respectable 3.91 ERA in 33 games. However, Koo will be forever remembered for his hitting and base running against Randy Johnson of the Yankees. In just his second Major League at-bat Koo doubled off the center field wall. But the fun was just getting started. Next batter Jose Reyes laid down a sacrifice bunt advancing Koo to third. But seeing that the defense had fallen asleep, Koo decided to keep going home and narrowly missed the tag by Jorge Posada to score.
Following the season, the Mets sold his contract to the Hanwha Eagles of the Korean Baseball Organization.
While with the Mets Koo wore number 17.