Financial terms of Witherspoon's contract weren't disclosed by the team.
Witherspoon played in 54 games for the Rams from 2006 to '09. The Rams were desperate for receiver help in 2009 when they traded Witherspoon to the Philadelphia Eagles for Brandon Gibson and a fifth-round choice.
He celebrated his return to St. Louis in a tweet Monday afternoon.
Good to be back #RAMS & looking forward to a great year
— WillWitherspoon (@willwitherspoon) July 22, 2013
Witherspoon, 32, played for current Rams coach Jeff Fisher and key assistants while with the Titans in 2010. He remained with Tennessee through 2012 and has seen his role change from every-down player to 34.5 percent of the snaps last season.
Jo-Lonn Dunbar and James Laurinaitis are firmly established as starting linebackers for the Rams this season. Rookie first-round pick Alec Ogletree is also expected to start. Josh Hull, a seventh-round pick in 2010, was the only other linebacker on the roster to enter the NFL as something other than an undrafted free agent the past two seasons. He owns one start and has played 40 snaps.
Information from ESPN.com NFC West blogger Mike Sando was used in this report.