A small amount of drugs and a stun gun were found when officers raided the empty bus, which was parked under the Globen concert venue in Stockholm where Bieber was performing Wednesday, according to police spokesman Lars Bystrom.
Authorities say they have no suspects and no one was arrested. Bystrom did not identify the drug, saying it had been sent to a lab for analysis.
The spokesman said police smelled marijuana coming from inside the bus while it was parked outside the hotel where Bieber was staying. The drug squad was notified and they searched the bus during the singer's concert.
Bieber is in Stockholm as part of a world tour.
The Associated Press contributed to this report.