Jerome Martin Koosman (born December 23, 1942) is an American former professional baseball pitcher.

He played in Major League Baseball (MLB) for the New York Mets, Minnesota Twins, Chicago White Sox, and Philadelphia Phillies between 1967 and 1985.

Koosman is best known as a member of the Miracle Mets team that won the 1969 World Series.

[1] Koosman was about to be cut from the Mets in 1966, when Joe McDonald, the assistant farm director, requested Koosman be retained at least until his first payday, as he owed the Mets money they had wired him after his car broke down en route to spring training.

That year, he was a member of the Mets team that unexpectedly won the National League East title (both leagues had split into two divisions after expanding from ten teams to 12) after finishing last in five of their first seven seasons, as well as trailing the Chicago Cubs for much of the season, and by as many as 91⁄2 games.

In Game Two of the NLCS against the Atlanta Braves, Koosman was shelled for six runs in 42⁄3 innings (including a Hank Aaron three-run homer).

The Mets regained the lead in the top of the ninth; Koosman got two outs in the bottom of the frame, then walked the next two batters.

He fell behind 3–0 in the third inning after giving up home runs to his mound opponent, Dave McNally, and Frank Robinson.

This victory gave the Mets a 3–2 lead in the Series; however, Oakland won the next two games to repeat as World Champions.

The remainder of the team deteriorated, especially Koosman who slumped to 8–20, finishing tied with Phil Niekro for most losses in the NL.

After a 3–15 season in 1978 Koosman, seeing no imminent improvement to the team, was traded to the Minnesota Twins at his request.

(Orosco had been the player to be named later who went to the Mets to complete the deal, which had been made two months earlier, in December 1978.)

On August 30, 1981, less than a month after the 1981 players' strike ended, the Twins traded Koosman to the Chicago White Sox.

[5] Koosman has since been joined by Keith Hernandez, Willie Mays and Dwight Gooden in having their numbers retired.