CLOSE 17-year-old rap sensation Chief Keef had $8,000 in CASH rolled up in his pocket at the time he was arrested for speeding in Illinois this week … this according to the police report obtained by TMZ.

Cops say Keef was driving a black BMW SUV at 110 mph when he was pulled over around 3:18 AM on May 27.

Keef’s due back in court on June 17 — and cops have reminded the rapper that since he’s a minor, he’s got to be accompanied by a parent or guardian.