Thornburg replaces cornerback Joey Thomas, who was released today.
Thornburg had joined San Francisco on Oct. 10 and posted four special teams tackles in two games played with the 49ers. He was waived by the team on Tuesday.
A native of Cathedral City, Calif., Thornburg initially signed with Philadelphia as a rookie free agent on April 29. He was inactive for the Eagles' first four games before being waived on Oct. 10.
The 6-foot, 196-pound defensive back led Northern Arizona in tackles his final two seasons. As a junior he garnered All-Big Sky honors after registering four interceptions.