Major League Baseball said Pineda tested positive for the banned substance and the suspension takes effect immediately.
Pineda, 6-foot-7 and 280 pounds, said in a statement he took an over-the-counter medication given to him by an acquaintance to help manage his weight. The pills contained hydrochlorothiazide, a diuretic that can mask other substances.
"Michael Pineda is a big member of this team in a lot of different ways, beyond the field as well as on it," Minnesota manager Rocco Baldelli said Saturday before the Twins held a team meeting preceding their game at home against Cleveland. "Because of that, it does create a challenge. Our team has been pretty resilient with everything that's been thrown at it to this point, and I think we're going to have the ability to acknowledge this and process what's going on and still continue to go out there and do our jobs."
Pineda apologized to the organization, teammates, family and fans for his "error in judgment" and said he "never intended to cheat the system, other players or opposing teams."
He said, however, he takes responsibility for "what goes in his body" and accepts the suspension. He added that he hopes to be an example to others about the importance of checking with experts before taking substances from an outside source.
Pineda originally was suspended for 80 games, but the ban was reduced to 60 on appeal, as a compelling case was made that the banned diuretic he used was not a masking agent for performance-enhancing drugs, a source told ESPN's Jeff Passan.
The suspension came one day after a strong start by Pineda against the Indians. Pineda, who went 11-5 with a 4.01 ERA, will miss the Twins' final 21 games plus any postseason games. Minnesota has a 5½-game lead over Cleveland in the AL Central after beating the Indians 5-3 Saturday. The Twins' magic number is 14; they've won 11 of 14.
"We still have a pretty decent lead in our division but still an opportunity to create some more space," reliever Sergio Romo said. "So, at the end of the day, it continues. This is a business but this is a game that it's just bigger than one person. So, good luck but also let's go. Win Twins. It doesn't change anything. Let's just play."
Pineda will forfeit $989,247 of his $8 million salary. He is eligible for free agency after the season and would serve the rest of the suspension next year.
The right-hander's presence in Minnesota's rotation has been one of the most underappreciated improvements made by the Twins this year. On Friday, in his 26th start of the season, Pineda reached double-digit strikeouts for the first time since April 10, 2017, with the New York Yankees. Pineda missed the second half of that season and, after signing with the Twins, all of 2018 while recovering from Tommy John elbow surgery.
In his past 14 starts, the big right-hander was 10-4 with a 2.96 ERA and 87 strikeouts in 82 innings.
"He's been our best pitcher in the second half. Definitely a big loss for us," designated hitter Nelson Cruz said, later adding: "When you see yourself going through this, it either can put you down pretty bad or you can step up. I think somebody has to step up for him."
Developing as one of the Twins' steadiest starters, Pineda likely would have been near the top of any playoff rotation for Minnesota. Baldelli wasn't sure how the team would fill Pineda's spot in the rotation and said the starts could come from several players. Pineda would have been in line for four more starts during the regular season.
"I think we have several ways to do it, and I think it could come down to each individual outing and maybe the specifics of that outing, and we'll make the decisions probably a day or two or three out from those games, at least in the short term," Baldelli said. "Committing to that kind of stuff, too, when there's no clear-cut answer to that question is something we'll probably hold off on. But we have several good options."
In 2014, Pineda was suspended 10 games while with the New York Yankees for putting pine tar on his neck to help get a better grip during a game in Boston.
Information from The Associated Press was used in this report.