Key (hip) signed with the Colts on Thursday, Amanda Foster of the team's official site reports.
The linebacker missed Week 18 of last season due to a hip injury, but he appears to be healthy again. Key played in 12 games last year for the Titans, recording 22 tackles (nine solo), including 4.0 sacks. He has finished with 4.0 or more sacks in each of the last five campaigns.