They promoted rookie running back Fitz Toussaint from the practice squad to the 53-man roster, according to a source.
If Cox had remained on the roster for the opening game, his $730,000 salary would have been guaranteed for the entire season as a vested veteran.
Cox being released suggests the Ravens feel good about the health of their cornerbacks, including starter Lardarius Webb. Webb is questionable with a lower back injury, but is expected to play after participating in practice all week. Cornerbacks Jimmy Smith (chest bruise, lung issues) and Asa Jackson (sprained ankle) aren't listed on the injury report.
Toussaint had an impressive game against the New Orleans Saints in the Ravens' preseason finale, rushing for a game-high 103 yards.
The Ravens will now have four running backs on the active roster, not including fullback Kyle Juszczyk, with running back Ray Rice suspended for the first two games for violating the NFL personal-conduct policy for his domestic violence felony aggravated assault case.
Bernard Pierce is slated to start in place of Rice with veteran Justin Forsett, rookie Lorenzo Taliaferro and Toussaint backing him up.
Toussaint auditioned for the Ravens at their rookie minicamp, earning a roster spot.
Toussaint played at Michigan, rushing for 648 yards and 13 touchdowns as a senior last season. He caught 18 passes for 203 yards for the Wolverines in 2013.
In 2012, he suffered a season-ending leg injury and was limited to 530 yards and five touchdowns.
Toussaint rushed for 1,041 yards and nine touchdowns in 2011 when he earned All-Big Ten Conference honorable-mention honors.
The 5-foot-9, 204-pound back ran the 40-yard dash in 4.49 seconds at his campus Pro Day workout and bench pressed 225 pounds 24 times.
As for Cox, he cut by the Ravens last Saturday, then re-signed on Tuesday when inside linebacker Josh Bynes was released.
Cox has now been cut four times this year, once by the San Diego Chargers one year after signing a four-year, $20 million contract, once by the Minnesota Vikings during the preseason roster cutdown to 75 players and twice by the Ravens.
Cox had three tackles and one pass deflection against the Saints.
Cox recorded 38 tackles and one interception last season for the Chargers in his lone season in San Diego. He was benched three times in San Diego and eventually lost his starting job to Richard Marshall.
The primary issue Cox had was struggling as a miscast slot cornerback in San Diego after having success outside with the Jacksonville Jaguars, who drafted him in the third round in 2009 out of William & Mary.
In his first NFL game in 2009, Cox intercepted a Peyton Manning pass and also recovered a fumble. He finished with 58 tackles and four interceptions as a rookie.