The move comes nearly three weeks after Detroit released two other players -- receiver Nate Burleson and safety Louis Delmas -- to dump their salaries.
The Lions are now more than $10 million under the cap.
How much they'll have to pursue top players to fill needs at receiver, safety and cornerback remains to be seen. They'll need to use some money to sign their own free agents and they also have to reach deals with their upcoming draft picks.
Another way to create more cap space would be to re-sign defensive tackle Ndamukong Suh to a contract extension and lower his current $22.4-million cap hit for 2014 in the process, but he still hasn't announced who will be representing him in those potential negotiations after firing his agent weeks ago.
Harris, 29, became another logical salary-cap casualty because the Lions can fill his role with younger players who cost less, including former practice-squad member Rodney Austin and a possible draft pick.
Harris spent only one season with Detroit and did not play in any games. His first six years were with the Tennessee Titans, who drafted him in the fourth round out of North Carolina State in 2007.
He suffered a knee injury in 2012, had some setbacks along the way in trying to make a comeback with the Lions and ended up being inactive for all 16 games last season.