Financial terms were not disclosed by the Rams but the 235-pound Jackson is widely expected to be one of the highest paid backs in the league.
The running back had been about to start the final season of a five-year, $7 million contract he signed as a first-round pick by St. Louis in 2004.
"I was never, ever, ever in doubt that this would get done," Rams head coach Scott Linehan told the team's official website (http://www.stlouisrams.com). "The timing now is very good for incorporation in the upcoming weeks for the opener."
The Rams are scheduled to play the Philadelphia Eagles in their season-opener on Sept. 7.
Although Jackson, 25, trained with his team mates on Thursday, Linehan said he was unlikely to play in Saturday's preseason game against the Baltimore Ravens.
"You can certainly work him physically to where he is getting himself in shape," Linehan said. "We have plans to do some extra things with him even when we are not practising to get him some extra time on the field."
Jackson rushed for 1,002 yards and five touchdowns for the Rams last season despite missing four games because of injuries.
