"After 11 years of having played the greatest team sport at its highest level, it's now time for me to announce my retirement from the NFL," Harris said in a statement on his agency's Twitter account.
Harris, 34, unceremoniously released by the Jets last June, signed a two-year, $5 million contract with the Patriots, hoping to win a Super Bowl ring.
That didn't happen, and Harris played a smaller-than-expected role. He appeared in only 10 games (177 defensive snaps) and was inactive during the team's run to Super Bowl LII.
Harris was due to count $2.75 million against the Patriots' cap, but his retirement will save them $2.1 million.
He will be remembered for his decade with the Jets, 2007 to 2016. Drafted out of Michigan in the second round, he played 154 games and finished as the franchise's second-leading tackler.
Harris never made the Pro Bowl, but he was regarded by coaches and teammates as the backbone of a once-formidable defense. In 2009, the Jets ranked No. 1 in total defense and fewest points allowed.
Harris said he was "blindsided" when he was released last offseason amid the team's massive roster purge. Despite the awkward ending, he's considered a strong candidate for the team's Ring Of Honor.
Former Patriots teammate Kyle Van Noy tweeted his support for Harris.
— Kyle Van Noy (@KVN_03) February 23, 2018
Van Noy said Harris "should already have his Green coat! Nobody played a decade like him of a true every down linebacker! Impressive player but even a better person!"