The move saves the Giants more than $5 million toward the 2018 cap.
Marshall finished the season with a career-low 18 catches for 154 yards and no touchdowns.
Marshall was reportedly "under the impression that he would remain on the Giants' roster" after agreeing to take a pay cut in March.
Marshall and Odell Beckham Jr. were expected to be one of the best wide receiver duos in football a year ago, but injuries ended any hopes of that happening.
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It's not as though the Giants are struggling at receiver following Marshall's departure.
Now, Marshall is entering free agency in mid-April, at the age of 34, and after consecutive lackluster seasons. The six-time Pro Bowler had eight seasons of 1,000 yards or more in the last 10, including a 1,502-yard campaign in 2015 in which he led the league with 14 receiving touchdowns. But recent success has eluded him. So, it doesn't seem like he'll retire just yet. I think I'm two great years away from, and I'll say it, I want to be a Hall of Famer and I think I got two great years to go to be mentioned with some of the greats.
Just a year ago, the Panthers were a touchdown away from advancing to the Divisional Round of the NFL Playoffs.
Bryant, meanwhile, already indicted interest in playing alongside ODB.