The add-on is worth $11 million, a source told Adam Schefter of ESPN.
Harrison reportedly now has $12 million guaranteed over 2019 and 2020. He previously had two years remaining on a contract he signed while a member of the New York Giants, but it had no guaranteed money remaining.
The 2016 first-team All-Pro skipped organized team activities and minicamp in an effort to get a new deal.
Harrison helped bolster Detroit's run defense last year after being acquired midseason from the Giants for a fifth-round pick.
The 30-year-old racked up 50 total tackles and 3.5 sacks in 10 games with the Lions.