Chief Keef Arrest -- Is That $8,000 In Your Pocket??

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.

During the arrest, cops say Keef explained, "I just got back from doing a show in Iowa, I'm a rapper."

Cops proceeded to put Keef in handcuffs and conducted a search. They found a bulge in his front left vest pocket -- which turned out to be a "large roll of dollar bills" ... $8,000 to be exact.

Cops say they informed Keef the money would not be removed from his pocket during the arrest.

Eventually, Keef was booked for excessive speeding and for driving with too many passengers in the car (Keef's permit only allows him to have 1 passenger, he had 3).

According to the police report, Keef is listed at 5'8" and 145 lbs.

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.