The move came only one day after Chicago placed two tackles, Anthony Adams (triceps) and Antonio Garay (leg), on injured reserve. They joined former starting tackle Dusty Dvoracek (knee) on the injured reserve list.
In addition, tackle Darwin Walker has been battling knee and elbow injuries.
A fifth-year veteran, Kennedy was the Rams' first-round choice in the 2003 draft, and he played his NFL rookie campaign under Lovie Smith, when the Chicago head coach was the St. Louis defensive coordinator. Kennedy, 28, was the 12th overall selection in the draft that year.
The former Penn State star was traded to the Denver Broncos this spring for a draft pick, but was released before the start of the season. Kennedy drew interest from a few teams during the season, but did not reach a contract accord until the Bears signed him. Details of his contract were not yet available.
Kennedy had battled weight problems and a foot injury in the past, but had a career-best 39 tackles in 16 games for the Rams in 2006.
In 53 appearances, Kennedy has 90 tackles, four sacks and one forced fumble.
Len Pasquarelli covers the NFL for ESPN.com.