Financial terms were not disclosed.
Spaeth spent four seasons with Pittsburgh before leaving as a free agent to sign with Chicago in 2011. He was cut by the Bears last week after two years with the club.
In 16 games last season, including eight starts, Spaeth had six receptions for 28 yards with a touchdown. He started 16 of the 31 games he played in two years with the Bears.
The Steelers selected Spaeth in the third round of the 2007 NFL Draft. The Minnesota product has 49 career receptions for 353 yards with eight touchdowns in 91 games.
