MLB said Laureano, a 27-year-old who is in his fourth season with the A's, tested positive for nandrolone, a performance-enhancing drug. The suspension is effective immediately.
Laureano denied that he knowingly took the banned substance in a statement released by the players' association and said he "was shocked" when he learned about the positive test.
"I have worked too hard and given too much to this sport to disrespect or cheat the game that I love," he said in the statement. "I would never do anything to dishonor my family, teammates, and coaches or do anything to let down the many young ball players that look up to me.
"I take great care of my body and have an extremely regimented diet. Based on the minuscule amount that was briefly in my body, I've learned that it is likely that it was contamination of something I ingested.
"I have been educated about PEDs through the RBI program and Baseball Factory while growing up. I know I don't need any of that to perform on the baseball field. All my athletic success has come from my hard work, focus and dedication to the game."
The penalty came down just over four hours before the A's were to begin a weekend home series against the Texas Rangers.
"The A's were disappointed to learn of this suspension. We fully support MLB's Joint Drug Prevention and Treatment Program and we will welcome Ramón back after the discipline has been served," the A's said in a statement.
Laureano is hitting .246 with 14 homers and 39 RBIs in 88 games this season. He has regularly made highlight-reel throws from center field to save runs.
"You're forced to just move on,'' A's manager Bob Melvin said. "Everybody feels bad for what happened for him. In baseball you have to have a short memory. There's only 50-something games left. We have to go out there and play with the pieces that we have. We still have good ones. He is a tough one to lose.''
Oakland, the reigning AL West champion having reached the playoffs in each of the past three seasons, is in second place behind the rival Houston Astros in the division race.
"I'd like to apologize for the distraction that this might cause my teammates, Billy Beane, David Forst, and the entire Oakland organization, community, and fan base," Laureano said. "I am devastated. Anyone who truly knows me as a person knows how much I love the game and that I would never intentionally do something like this."
Laureano became the fifth player suspended this year under the major league drug program after Miami pitcher Paul Campbell, Colorado third baseman Colton Welker, San Francisco pitcher Daniel Santos and Seattle pitcher Hector Santiago.
"I think everybody was shocked,'' Melvin said. "These things aren't fun. Ramon's beloved in our clubhouse. They have to abide by the rules and everybody knows them. Unfortunately we're in the position that we are right now.''
The Associated Press contributed to this report.