"He's a veteran pitcher who can help our staff in a variety of ways," Mets GM Omar Minaya said.
"I'm excited about the chance to pitch in the major leagues," said Koo, who switched throwing hands in high school. "It's been my dream to face the highest level of competition possible. I'm thankful to get this opportunity."
Over his four-year Japanese League career, Koo was 24-34 with 10 saves and a 3.88 ERA in 110 games. He defeated Japan in the 2000 Olympics in Sydney, Australia, helping Korea to a bronze medal.
Koo also pitched in the Korean League from 1993 to 2000, going 61-57 with 150 saves and a 2.79 ERA in 351 games. He won a MVP award in 1996, posting a league-best 1.88 ERA and 18 victories while also winning a Gold Glove and collecting 24 saves.