The 10-year veteran spent the past three seasons with the Carolina Panthers. He previously played for the Indianapolis Colts, Tennessee Titans, Atlanta Falcons and New York Jets. He has 31 catches for 312 yards and one touchdown in 131 games. Last season he played in 13 games with no receptions.
Hartsock was one of three tight ends signed by the Patriots on Sunday along with Steve Maneri and Terrence Miller.
The Patriots need depth at the position with Rob Gronkowski practicing but not participating in contact drills as he recovers from knee surgery and Michael Hoomanawanui sidelined for an undisclosed reason.