Justin Bieber was arrested for DUI ... TMZ has learned.
Law enforcement tells us ... Bieber was busted in Miami Beach for both DUI and drag racing.
The arrest occurred early Thursday morning.
Bieber is now in custody and is at the police department where he is being tested -- presumably to more precisely measure his blood alcohol level.
His next stop will be jail, where he will be booked and processed and bail will be set.
Based on what we know ... the traffic stop was for drag racing and cops determined he was driving under the influence.