The team officially listed the move as "failed physical" among the league's transactions, as Kiwanuka is still recovering from knee surgery.
Kiwanuka was scheduled to earn $4.825 million in salary and bonuses this year and count $7.45 million against the salary cap. Releasing him saves the Giants $4.825 million against this year's cap and leaves Kiwanuka free to sign with any team, even before free agency begins March 10.
Kiwanuka's production tailed off the last couple of years, but he was long seen as a valuable Giant. He was the No. 32 overall pick in the 2006 NFL draft -- the last one for which Ernie Accorsi was the Giants' general manager -- and switched between defensive end and linebacker depending on the team's year-to-year needs. He agreed to rework his contract several times, took a pay cut last year and devoted a great deal of his time on the practice field and in the locker room to helping the team's younger players.
"The thing that has always impressed me about Kiwi is how serious he is about the game," Giants coach Tom Coughlin said in the team's news release. "He was always prepared and always in outstanding condition and played very hard. And he had some nasty in him. I always felt like you could count on certain things from him: eight [or] nine sacks, harass the quarterback, and you could move him around, inside, outside. He is dependable, reliable and there is no question about his professionalism. His character and professionalism and serious approach to the game will be missed."
Kiwanuka amassed 38.5 sacks in his Giants career. His departure leaves quarterback Eli Manning and long snapper Zak DeOssie as the only remaining Giants from the 2007 team that upset the undefeated Patriots in Super Bowl XLII.