In a statement Saturday, MLB said Tejada tested positive for amphetamines and the suspension of the 2002 American League MVP is effective immediately.
Tejada -- who will not appeal the suspension -- said in a statement released through the MLB Players Association he used a banned substance but it was for treatment of a medical condition.
"I apologize to my teammates, the Royals organization and to the Kansas City fans," the statement said. "I have a medical condition that requires medication to treat. I took that medication while re-applying for a Therapeutic Use Exemption. Under the requirements of the Joint Drug Program, I made a mistake in doing so."
Tejada, 39, was batting .288 with three homers and 20 RBI for the Royals this season.
He will miss the remaining 41 games of the 103 season and then sit out the first 64 games in 2014.
Tejada pleaded guilty to lying to Congress in 2005 about his knowledge of performance enhancing drugs in baseball.
In 2,171 major league games with Oakland, Baltimore, Houston, San Diego, San Francisco and Kansas City, Tejada has a .285 batting average, 307 home runs and 1,302 RBI.
He was a six-time all-star and was named All-Star Game MVP in 2005.