The judoka Mohammad Samim Faizad, one of only six Afghans at the Paris Olympics, was provisionally suspended on Friday after failing a doping test for a banned steroid.
The 21-year-old, who was beaten 11-0 in the French capital on Tuesday in the men's -81kg category, had told AFP on the eve of the Games that competing at the Olympics was a dream come true.
The International Testing Agency said: "The ITA reports that a sample collected from judoka Mohammad Samim Faizad from Afghanistan has returned an adverse analytical finding for the non-specified substance stanozolol metabolites."
The sample was collected by the ITA on the day Faizad lost to Austria's Wachid Borchashvili in his opening bout.
"The athlete has been informed of the case and has been provisionally suspended until the resolution of the matter," the ITA said, adding the athlete has the right to request analysis of the B-sample.
There are three men and three women from Afghanistan at the Games.
Of the six, Olympic debutant Faizad was the only one who trained for the Games inside his Taliban-controlled homeland.