Byrd is currently a free agent, so he will be placed on the restricted list for the duration of his suspension, which will begin immediately and will be in effect through Aug. 20. It will result in the loss of 50 days of pay.
The outfielder, an All-Star with the Cubs in 2010, tested positive for Tamoxifen, the Commissioner's Office said.
Byrd said the positive test happened because of medication he was taking for a years-old condition that is unrelated to baseball and recurred during the offseason. By
"Although that medication is on the banned list, I absolutely did not use it for performance enhancement reasons," Byrd said. "I am mortified by my carelessness and I apologize to everyone who loves this game as I do. I will serve my suspension, continue to work hard and hope that I am given an opportunity to help a Club win later this season."
Byrd, 34, has had a long-standing relationship with Victor Conte, who pleaded guilty to distributing steroids as part of the investigation of his company, Balco, in 2005. Conte served four months in prison and four months of home confinement, but Byrd said last year that he wasn't concerned about the relationship.
Byrd told reporters in February 2011 that he has used supplements since college, but was not concerned about testing positive for any banned substances, and that he trusted Conte would never provide an athlete with anything illegal.
"To be honest, he could teach me how to beat the system if he wanted to, but I would have to ask him and then he would have to put himself in that situation again," Byrd said. "I don't want to do that, and he doesn't want to do that.
"He's not going to make a mistake with the supplements, and that's why I don't have to worry about it."
Added Byrd, at the time, of his relationship with Conte: "I'm sure the Cubs knew. They wouldn't have signed me if there were any worries. I'm a supplement guy. The Phillies knew it when I was drafted. I looked the same way."
Conte took to Twitter to deny his involvement.
Byrd was a 10th-round pick of the Phillies in the 1999 Draft, and made his Major League debut with Philadelphia in 2002. He finished fourth in the National League Rookie of the Year voting in 2003, his first full big-league season, when he hit .303 with seven homers, 45 RBIs, 86 runs and 11 stolen bases.
After four years with the Phillies, he also spent time with the Nationals (two years), the Rangers (three years), the Cubs (three years) and the Red Sox. He split time between Chicago and Boston this season -- a total of 47 games -- batting .243 with one home run and nine RBIs.
"I'm surprised and feel bad for him because he was a great teammate," said Cubs left fielder Alfonso Soriano. "I feel sad when you hear names and drug tests, no matter who's the guy, whether he's a bad or good teammate, you feel bad because he's part of the family."
Byrd was traded by the Cubs to the Red Sox in April and was released by the Red Sox on June 12. He began the season going 3-for-43 for the Cubs, for whom he played two-plus seasons. He was batting .270 in 34 games for the Red Sox when Boston released him.
"He played here, he played well, I had no indication," said Red Sox manager Bobby Valentine. "I don't think anybody here did."
Byrd is a career .278 hitter with 1,067 hits in an 11-season Major League career that has also included time with Phillies, Nationals and Rangers.