To make room for McGehee, the Marlins placed infielder Jeff Baker on the disabled list Friday because of a left intercostal strain.
McGehee won the NL Comeback Player of the Year award with the Marlins last year, batting .287 with 76 RBIs. They then traded him to the Giants, but he struggled this year and was batting only .213 when he was designated for assignment June 30.
McGehee, 32, provides depth at third base while Martin Prado completes his recovery from a shoulder injury. He's expected to return next week.