Ridley joined the Steelers in December after fellow running back James Conner was placed on injured reserve with an MCL injury. He rushed 26 times for 108 yards and a touchdown in two games.
The 29-year-old Ridley is expected to compete with Fitzgerald Toussaint for playing time. Ridley also played with the Minnesota Vikings in 2017 after having brief stints with the Detroit Lions, Indianapolis Colts and Atlanta Falcons the previous season.
Ridley was drafted by the New England Patriots in 2011 and enjoyed his best season in 2012, when he had 290 carries for 1,263 yards and 12 touchdowns. Ridley spent four seasons with New England, but tore his ACL in 2014.