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.

Read more: http://www.tmz.com/2015/08/24/young-...#ixzz3jm3PEiY1

http://www.bet.com/news/music/2015/0...test-fail.html