Defensive tackle Darell Scott was released Thursday to make room for Clemens, who understood he'd be the backup behind Sam Bradford on Sunday against the Washington Redskins in the home opener. Rookie Austin Davis was the backup for the opener.
The 29-year-old Clemens has a home in St. Louis and stayed in town after getting released with the understanding the Rams would add him to the roster when they could.
Clemens made three starts last year, completing 52.7 percent of his passes for 546 yards with two touchdowns and an interception.
Scott played in 13 games with one start last season. He was a fourth-round pick in 2009.