Young Buck Urine Big Trouble Now

8/24/2015 12:30 AM PDT BY TMZ STAFF

0821-young-buck-INSTAGRAM-01Young Buck could go to prison for faking a piss.

The rapper -- who is on a probation -- used someone else's urine to pass a drug test ... so claims his probation officer.

The P.O. says Buck came in for his drug test and concealed a plastic bag with someone else's clean pee, and then used a tube to transfer it into a sample cup.

Turns out it's a felony to falsify a court-ordered urine test, punishable by up to 5 years in prison.  

According to court docs, Buck had been smoking weed while traveling with another rap artist. TMZ broke the story ... that artist is none other than Snoop.