The suspension, effective immediately, was handed down after the hurler was detected with Clenbuterol in his system, the baseball commissioner's office said.
Mota, a 14-year veteran, was suspended for 50 games in 2006 after testing positive for PEDs.
"The Giants are disappointed to learn of Guillermo Mota's suspension," the Giants said in a statement. "We support Major League Baseball's drug prevention and treatment program."
Mota had gone 0-1 with a 5.06 earned run average in 10 2/3 innings this season.
After breaking in with the Montreal Expos [now the Washington Nationals] in 1999, Mota went on to pitch for the Los Angeles Dodgers, Florida, Cleveland, the New York Mets and Milwaukee, before joining the Giants in 2010. He has a lifetime record of 39-45 with an ERA of 3.92.