Young was injured Sunday. Physicians diagnosed the problem as a sprained left knee and said Young would miss 2-4 weeks.
Kerry Collins has been made the Titans' starter while Simms, who was with the Tampa Bay Buccaneers for five seasons but only appeared in 19 games, will serve as his backup.
"Chris has been a starter in the league and should fit into our system well," said Titans General Manager Mike Reinfeldt. "He is a smart player, who is a student of the game and has the presence in the huddle that you look for in quarterback."
Simms started 15 of the 19 games in his NFL career but hasn't played since he suffered a ruptured spleen in a game early in 2006. He has thrown for 3,087 yards and 12 touchdowns in his career.
The Titans also said they waived tight end Dwayne Blakely to make room on the active roster for Simms. Tennessee also added quarterback Preston Parsons to its practice squad.