Details of the contract were not disclosed, but multiple media outlets reported Johnson agreed to a one-year deal.
In a corresponding move, the Raiders waived defensive back Darius Hillary, who had been signed to a futures contract by the Raiders in January after playing in one game for the Cleveland Browns in 2017.
Johnson joins the Raiders after 13 seasons with the Kansas City Chiefs, a longtime archrival of the Raiders.
Despite tearing his left Achilles tendon for the second time in two years late in the 2016 season, the 35-year-old Johnson came back to make 71 tackles and break up seven passes in 15 games for the Chiefs last season.
Johnson, a four-time Pro Bowl selection, is expected to provide a veteran presence to the Raiders' defense in addition to upgrading pass coverage at the second level.
The signing of Johnson might indicate that the Raiders will not re-sign veteran linebacker NaVorro Bowman, even though Raiders general manager Reggie McKenzie has said he would like to keep the former San Francisco 49er.
Kansas City selected Johnson in the first round of the 2005 NFL Draft (15th overall), and he has played in 182 NFL games with 169 starts. He has totaled 1,262 tackles (993 solo), 27.5 sacks, 23 forced fumbles, eight fumble recoveries, 14 interceptions and 77 passes defensed. In the postseason, Johnson has played in six games and posted 40 tackles (26 solo) and one sack.
He is the Chiefs' all-time leader in tackles. He was named a first-team All-Pro in 2011 after setting a franchise record with 179 tackles, 4.5 sacks and two interceptions.