Lil Twist was driving Justin Bieber's car while under the influence of marijuana ... and he even had a LIT BLUNT in the car at the time of his DUI arrest, law enforcement sources tell TMZ.
We broke the story ... 20-year-old Twist was busted in Calabasas around 3:30 AM Thursday morning on his way to Bieber's house.
Twist was hauled to a nearby station, where he was booked ... and was later released after roughly 8 hours in custody.
As he left the Sheriff's station, Twist didn't have much to say to us. We're guessing he'll be more talkative when he explains himself to JB.