According to Stefan Stevenson of the Fort Worth Star-Telegram, Bonifacio tested positive for Boldenone.
The 24-year-old had a productive rookie season with the Royals in 2017, as he hit .255 with 17 home runs and 40 RBI in 113 games.
In spring training this year, Bonifacio was making a strong case to be a middle-of-the-order bat with a .333 batting average, one homer and seven RBI.
Bonifacio was penciled in as Kansas City's starting right fielder, but it will be forced to consider other options.
Moving Jorge Soler from designated hitter to right field is a possibility, as is playing one of Jon Jay or Paulo Orlando at the spot while the other mans center field.
The Royals lost two of their biggest offensive contributors in free agency, as Lorenzo Cain joined the Milwaukee Brewers and Eric Hosmer signed with the San Diego Padres.
Kansas City is looking to bounce back in 2018 after going 80-82 and missing the playoffs by five games last season.