A state police spokesperson said Bieber, while "horsing around with members of his staff," threw water balloons at two troopers, NBC affiliate WBAL reported. One of the balloons hit a trooper's gun belt and burst, and the other bounced off a trooper's chest, the station reported.
Bieber was outside his trailer before a concert Sunday and the troopers were on hand for crowd control. A state police spokesperson said it was "inappropriate behavior" by Bieber but said no further action would be taken by police.
The 16-year-old Bieber performed in front of a sold-out crowd Sunday night at the Timonium Fairgrounds. Tickets sold out less than 20 minutes.
The Canadian performer's debut single, "One Time," was released worldwide in 2009. His first studio release, "My World 2.0," was released in March.
Bieber's ticked off the cops before: Last fall he appeared at a mall on New York's Long Island to sign CDs and a melee broke out in the crowd. A cop was injured and cops arrested an Island Def Jam Records exec who failed to cancel the event after it got out of hand.