Terms were not disclosed.
Harnish was a seventh-round draft pick out of Northern Illinois by Indianapolis in 2012. He split his rookie season between the active roster and practice squad with the Colts and was on the practice squad all of 2013.
Harnish spent last season on Minnesota's practice squad, appearing on the active roster but not playing against Green Bay on Oct. 2.
He threw for 8,944 yards and 68 touchdowns in four seasons with Northern Illinois.