The compensation will be a "flip of late-round picks," per Ian Rapoport of NFL Network.
Harmon started eight games last season and has appeared in all 16 games in each of the last six years. He finished 2019 with 22 tackles, two interceptions and five passes defended.
Despite minimal stats, Harmon has played a significant role in the Patriots defense, per ESPN's Mike Clay:
New England had the No. 1 scoring defense in the NFL last year, as it allowed the second-fewest passing yards and the fewest passing touchdowns.
Harmon was a key part of that success, playing 65 percent of defensive snaps while allowing just a 39.7 quarterback rating on 15 targets in his direction, per Pro Football Reference.
He will now try to translate that production with the Lions, who finished 2019 dead last in the NFL against the pass.
Detroit head coach Matt Patricia also knows Harmon well as the former Patriots defensive coordinator. He previously worked with the 29-year-old from the time he entered the league in 2013 until 2017.
The Lions additionally signed former Patriots linebacker Jamie Collins on a three-year deal, per Rapoport, clearly trying to build a team with players Patricia trusts.