The urine sample was collected from Choi before March 28, when Major League Baseball and the players' union increased the penalty for a first testing violation for PEDs to 80 games. MLB said Thursday that Choi tested positive for methandienone, a controlled substance popular with bodybuilders.
The 22-year-old Choi was hitting .394 at Triple-A Tacoma with one homer and five RBIs.
Choi became the third player suspended this year under the big league program following Yankees star Alex Rodriguez and Rays pitcher Alex Colome.
''I do not know what I could have taken that caused me to test positive,'' Choi said in a statement issued by the union. ''However, I also understand that without an explanation I must serve a suspension and I accept that.''
Seattle said it was ''disappointed.''