The commissioner's office announced the drug penalty on Wednesday. The ban will take effect at the start of next season.
Grandal made his big league debut this year. He hit .297 with eight home runs and 36 RBIs in 60 games. He turns 24 on Thursday.
Grandal issued a statement in which apologized to fans, his teammates and the Padres. He said he took responsibility for his actions.
There have been seven suspensions this year under the major league drug program. There have been 101 minor league suspensions.