Young 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.