Justin Bieber DUI -- He Was High on Weed and Pills ... Toxicology Report Shows

The Justin Bieber toxicology report from his DUI arrest in Miami Beach is in ... and it shows he tested positive for marijuana and prescription medication, TMZ has learned.

According to the report -- obtained by TMZ -- Justin's urine was tested right after he was busted for driving under the influence on January 23 ... and it shows there was THC and Alprazolam in his system.

FYI -- Alprazolam is the key ingredient in Xanax (prescription anti-anxiety medication).

The report also shows Justin tested NEGATIVE for cocaine, opiates, meth and other drugs.

The report is consistent with the police report where cops say he told them his mom had provided him with prescription pills.

As for the weed, Justin also told cops he had been smoking pot in his recording studio all night long.

Bieber has pled not guilty to the charges.