The deal is for five years and $45 million, reported ESPN's Ed Werder, who added that he will remain a linebacker.
Barron isn't a linebacker by trade. At 6'1", 213 pounds, he came into the league with the Tampa Bay Buccaneers as a safety before he was traded to the Rams during the 2014 season.
Upon his arrival to St. Louis, he played games at safety, cornerback and linebacker. He was thrust into the linebacker role during the 2015 season when outside linebacker Alec Ogletree went down with injury.
He made an immediate impact at linebacker in Week 6 against the Browns, as head coach Jeff Fisher told ESPN.com's Nick Wagoner: "The defensive staff had him credited with 19 tackles, two caused fumbles and two tackles for loss. He’s filled in nicely. ... We miss Alec, but we’ve adjusted with Mark. ... Mark’s just going to continue to get better."
For Barron, it didn't matter where he was placed in the defense.
"I’m just playing football," Barron told Wagoner. "I’ve been playing football for a long time, so you can line me up in a lot of places and I can be a good football player for you."
Barron recorded a career-high 113 combined tackles last year, which is why the Rams would plan to keep him at the position even with Ogletree back and healthy, according to Wagoner:
The spot at middle linebacker opened up when the team released seven-year veteran and franchise tackling leader James Laurinaitis on Friday.
Now Barron will remain with the Rams in his new position for the foreseeable future.