Koch has struggled with control and consistency since recording 164 saves over his first four major league seasons.
The right-handed closer, who was acquired by the Marlins from the Chicago White Sox for minor league infielder Wilson Valdez in June, went 1-2 with a 3.51 ERA and no saves in 23 games with Florida last season. He walked 20 and struck out 25 in 25 2/3 innings.
Koch was regarded as one of baseball's top closers after compiling an 11-4 record with a 3.27 ERA and 44 saves for the Oakland Athletics in 2002. It was his fourth consecutive season with at least 30 saves; it was also his last.
Traded to the White Sox in a deal involving Keith Foulke in December 2002, Koch managed just 19 saves and a 5.65 ERA in 79 games -- and lost his job as closer twice -- before being dealt to the Marlins.
Koch, who will turn 30 in December, was selected fourth overall by the Toronto Blue Jays in the 1996 draft. He posted 31 saves and a 3.39 ERA as a rookie with the Blue Jays in 1999 and added 69 saves over the next two seasons before being dealt to Oakland.
Also on Monday, the Marlins announced that catcher Ramon Castro has opted for free agency after being sent outright to Triple-A Albuquerque of the Pacific Coast League.
The 28-year-old Castro batted just .135 with three homers and eight RBI in 2004 before being placed on the 60-day disabled list with inflammation in the big toe on his right foot.