Cox, 27, was cut by Minnesota earlier this week as teams trimmed their rosters to 75. He joined the Vikings in March after being released by San Diego, which had signed him to a four-year, $20 million deal the previous year.
The William & Mary product lost his starting job with the Chargers, finishing with 38 tackles and one interception in 16 games.
A third-round pick by Jacksonville in 2009, Cox has registered 241 tackles and 13 interceptions while starting 56 of the 63 career games in which he has played.
The Ravens waived rookie wide receiver Jace Davis to clear room for Cox on the roster.