Levi Damien of SB Nation reported, per Yahoo Sports' Terez Paylor, that Johnson's deal is for one year and $3 million.
Johnson, 35, spent the first 13 years of his career with the Kansas City Chiefs.
Coming off the second torn Achilles of his career, Johnson registered 71 total tackles and seven pass breakups while playing 76.8 percent of the Chiefs' defensive snaps a season ago. That mark ranked second among all linebackers on the team behind Justin Houston.
By adding Johnson to the fold, the Raiders may be signaling a willingness to play free-agent signee Tahir Whitehead on the weak side.
If that's the case, Johnson would make sense as a NaVorro Bowman replacement at middle linebacker who can clog running lanes on early downs.
Johnson's arrival also means the Silver and Black have now signed seven players older than 30 years old in free agency as new head coach Jon Gruden continues to eye an expedited route to Super Bowl contention.