The eight-time All-Star second baseman tested positive for furosemide, a diuretic better known as Lasix, which is often used to help mask banned substances in urine tests.
Cano just went on the disabled list with a fractured right hand after getting hit by a pitch Sunday in Detroit, and he is expected to undergo surgery Wednesday in Philadelphia. But he'll now be out until his suspension concludes in mid-August, and he will not be eligible for the postseason.
Cano will forfeit nearly $12 million from his $24 million annual salary. The 35-year-old will still have five years and $120 million remaining on that contract after this season.
The Mariners aren't used to being without Cano, who has missed only 38 of 1,821 games since 2007 and joined Seattle prior to the 2014 season.
"It's definitely a tough one," designated hitter Nelson Cruz said. "It's sad news for everybody. We know how important he is for us and the organization. He's a good friend, so definitely we feel bad right now. We support him and wish him the best."
General manager Jerry Dipoto said he learned Monday that Cano was being suspended, and the club will move converted center fielder Dee Gordon back to second base and look for help outside the organization to bolster the roster, which suddenly is minus one of its biggest pieces.
But a Mariners team fighting to end a 16-year playoff drought will need to regroup, without question.
"Disappointment," Dipoto said of his first reaction. "We are all disappointed. We just lost one of our best players. It's a hit. It's a hit to Robbie. It's a hit to our club, to the franchise in general and to baseball. This is one of the great players in the game.
"It's important to know that it's hurtful to our fans. It's one of those things that really leaves an impression. I felt that disappointment. I think you all have figured out that I'm an optimistic person by nature. And it turned into, 'All right, how do we turn this to as positive as we can and solve the problem?' And that's what I've been thinking about ever since."
Dipoto said Cano "feels terrible" about the situation, and he asked the second baseman to convey his apologies to the team.
"Recently I learned that I tested positive for a substance called furosemide, which is not a performance-enhancing substance," Cano said in a statement issued by the MLB Players Association. "Furosemide is used to treat various medical conditions in the United States and the Dominican Republic. This substance was given to me by a licensed doctor in the Dominican Republic to treat a medical ailment. While I did not realize at the time that I was given a medication that was banned, I obviously now wish that I had been more careful.
"For more than 15 years, playing professional baseball has been the greatest honor and privilege of my life. I would never do anything to cheat the rules of the game that I love, and after undergoing dozens of drug tests over more than a decade, I have never tested positive for a performance-enhancing substance for the simple reason that I have never taken one."
Cano is hitting .287/.385/.441 with four homers in 169 plate appearances this season. He is fifth among active players with 2,417 career hits, and he is in the fifth year of a 10-year, $240 million contract he signed with Seattle during the 2013-14 offseason.
"We were disappointed to learn today that Robinson had violated the terms of Major League Baseball's Joint Drug Prevention and Treatment Program. Our organization fully supports the program," the Mariners said in a statement. "Robinson made a mistake. He has explained to us what happened, accepted the punishment and has apologized to the fans, the organization and his teammates.
"We will support Robinson as he works through this challenge."
Dipoto said the expectation was that Cano would only miss about four weeks with his hand injury, though that is now moot because he'll serve the suspension concurrently. Cano will be allowed to use the Mariners' facilities to rehab and work out during the suspension, but he will not be permitted to be with the team when the clubhouse is open or during games.
"Today I decided to accept MLB's suspension," Cano said. "This was the most difficult decision I have ever made in my life, but ultimately the right decision given that I do not dispute that I was given this substance. I apologize to my family, friends, fans, teammates and the Mariners organization. I am extremely grateful for the support I have received during this process, and I look forward to rejoining my teammates later this season."
Cano has been tested at least once since the positive test occurred, a source told MLB.com's Mark Feinsand, with nothing showing up on the test.