A police officer on crowd duty smelled marijuana on an empty tour bus outside the hotel where Bieber was staying just before his concert in the capital on Wednesday night, police spokesman Kjell Lindgren said.
Police searched the empty bus after it had taken 10 to 15 individuals to the concert venue.
"The police went onto the bus and searched it and found a small amount of narcotics," Lindgren said. "We don't know who had the drugs or who smoked them, so it will be hard to link them with any individual."
The drugs have been sent for analysis and Lindgren said the police did not plan further action unless they got more information.
Bieber is travelling Europe on his "Believe" tour and is due in Finland for a concert on Friday.
Bieber got into trouble earlier this month after a museum dedicated to Anne Frank said the 19-year-old had written that he hoped the young Holocaust victim would have been a "belieber", the word used by his fans to describe themselves.
Anne Frank, who died aged 15 at the Bergen-Belsen concentration camp in 1945, is one of the best-known Jewish victims of the Holocaust due to her diary.
