Kiwanuka was entering the last year of a two-year deal he signed last summer as a free agent, coming off a neck injury that forced him to miss 13 games in 2010.
He had a career-high 84 tackles and 3½ sacks last season and showed he has fully recovered from a herniated disc in his neck. The contract extension is worth $21.75 million, according to reports.
While Kiwanuka received a new deal, the Giants are still facing defensive end Osi Umenyiora's request for a contract extension. Umenyiora has been seeking a new contract for years and sat out the first two weeks of camp before later opting for minor knee surgery last summer in hopes of landing a new deal.
On Wednesday, Umenyiora said he would like to remain a Giant but could not see how the team could afford to give him the extension he wants and take care of younger players such as Kiwanuka, Victor Cruz, Hakeem Nicks and Jason Pierre-Paul.
Umenyiora, 30, suggested the Giants should trade him now while they can get something back in the NFL draft. The defensive end is entering the final year of his deal worth just under $4 million and does not want to play out the last year under his current contract.
General manager Jerry Reese has said he's open to an extension. But both sides are not close to one.
Umenyiora refused to speculate on how the Kiwanuka deal will affect his contract situation.
"No idea what it means, but we will see," Umenyiora said in an email to The Associated Press on Friday. "I thought it was a good, smart decision by the Giants."
Kiwanuka's extension solidifies the Giants on the defensive line in case Umenyiora does not return this season. Kiwanuka potentially could see more snaps on the line this season with the addition of linebacker Keith Rivers and the departure of backup defensive end Dave Tollefson in free agency.
Information from The Associated Press contributed to this report.