Spaeth, who was selected by the Steelers in the third round of the 2007 NFL Draft, underwent offseason knee surgery. He leaves the franchise with 55 receptions for 420 yards and 10 touchdowns in 123 regular-season games, and was part of the Steelers' Super Bowl XLIII championship team.
"Matt never fully recovered from knee surgery that occurred this past offseason, and he will not be ready to return to football activities at this time," said Steelers general manager Kevin Colbert. "Matt played an integral role in our Super Bowl XLIII Championship run, and we wish him the best in his future endeavors."
Spaeth's departure continues an overhaul to the tight end position that began with Heath Miller's retirement. The Steelers signed Ladarius Green in free agency to pair with second-year pro Jesse James. They also added David Johnson after his release by San Diego and have 2015 practice squad player Xavier Grimble and Paul Lang, who was signed June 7.