St. Louis is inheriting the final two years of Titus Youngs rookie contract at little risk. The move reunites Young with his receiving teammate at Boise State, Austin Pettis.
The Lions cut Young, a former second-round draft pick, when rosters unlocked after the Super Bowl and after a season of misbehavior.
The Rams were the lone team to place a claim on Young. The Lions hoped to trade Young before the April draft, but there was no market for the receiver.