Clayton, 28, was a first-round pick out of the University of Oklahoma, was with the Ravens for five seasons and started at least 10 games every season he was in Baltimore. He had 234 catches for 3,116 yards and scored 14 touchdowns, including two rushing scores.
His best season came in 2006 when he caught 67 passes for 939 yards and five touchdowns. In 14 games a year ago, including 12 starts, he had 34 catches for 480 yards and two touchdowns.
Clayton became expendable when the Ravens reached an agreement with WR T.J. Houshmandzadeh. Clayton will be playing under terms of the restricted free-agent tender of $2.277 million that he signed with the Ravens.
Ravens general manager Ozzie Newsome said of Clayton, "You never reach a point when it's easy to release or trade a player you respect. Everyone here has the highest regard for Mark. He'll help the Rams, and it's good for him that they reached out to get him.
"Mark is a good person who did everything he could to help the Ravens on and off the field. In the end, we have to do what we think is best for the team, but letting Mark go hurts. He was first class all the way, even when we met with him about the trade. This is a tough part of the business."
Clayton hasn't played or practiced since suffering what was described as a "mild" concussion in the Ravens' Aug. 28 preseason game against the New York Giants.
The deal also involves a swap of draft positions in an undisclosed round of the 2011 draft.