EXCLUSIVE

Stacey Koon -- the former LAPD sergeant who went to prison for his role in the Rodney King beating -- has been charged with DUI ... TMZ has learned.

Law enforcement sources tell us the L.A. County District Attorney's Office formally filed charges against Koon after he was arrested in May. He's facing 2 counts of misdemeanor DUI.

We broke the story ... Koon was busted after hitting a parked pickup truck. Cops who responded to the scene suspected he was under the influence of alcohol, and cited him on the spot. We're told his blood alcohol was almost twice the legal limit.

Koon was acquitted in the original King beating case -- which sparked riots in L.A. -- but was later found guilty in federal court for violating King's civil rights. He served 30 months in prison.