The league said Alfonzo tested positive for a performance enhancing substance in violation of MLB's Joint Drug Prevention and Treatment Program.
The suspension of Alfonzo, who was also banned 50 games in 2008 after a positive test while with the San Francisco Giants, is effective immediately.
The 32-year-old Venezuelan signed a minor league contract in January with Colorado, his fourth team in six years.
Alfonzo, who last played for the Rockies on Tuesday, was batting .267 with one home run and nine RBIs in 25 games this season.
Earlier this year, 12-time All-Star Manny Ramirez retired abruptly after being notified of a drug-policy issue rather than face a 100-game suspension.
(Editing by Frank Pingue)