Johnson would have been able to negotiate with other teams in free agency. His signing elsewhere would have granted the Rams two compensatory first-round picks.
The 26-year-old recorded 58 tackles in 2015 with seven interceptions. He will be paid $13.952 million with the tag.
With the move, 27-year-old corner Janoris Jenkins will enter free agency. Jenkins started opposite Johnson in 2015 and recently fired his agent heading into free agency.