Ridley, 28 was a free agent having been released by Cheltenham Town in the summer.
The Scunthorpe-born player came through the ranks at Glanford Park and made 117 appearances for the Iron.
He turned down an offer to stay in 2007, thinking he would not be a regular first-team player following promotion to the Championship as League One champions.
He spent three seasons at Whaddon Road, making 70 appearances in all competitions. He also had loan spells at Darlington and Lincoln City.