The A's announced the move Friday. The 38-year-old first baseman had returned to Oakland this season after seven years with the New York Yankees.
Giambi, the 2000 AL MVP, has been on the disabled list since July 20 with a strained right quadriceps muscle. His legs have been bothering him for months.
A five-time AL All-Star, Giambi was batting just .193 -- lowest in the majors when he went on the DL. He has 11 home runs and 40 RBIs in 83 games.