Diddy's Inmate Costume From 'Monster's Ball' Up for Auction

Diddy's prison outfit is going up for auction ... no, not one he's worn since being locked up in jail last fall, but the one he wore on set 25 years ago!

The rapper's iconic costume from his role as Lawrence Musgrove in the 2001 romantic-thriller drama "Monster's Ball" is going up for sale. It features a gray hoodie, jeans -- with one pant leg cut off -- and black shoes.

Diddy wears this outfit during the early parts of the film before his character is executed ... and, it's expected to fetch a decent price at auction when bidding starts on June 7.

The duds also come with a production costumer's tag that details some of the items and scenes in which they were used. The auction house says it acquired the costume from Star Wares of Los Angeles and it includes a Letter of Provenance.

Halle Berry, Billy Bob Thornton and the late Heath Ledger also starred in the flick, which netted Berry an Academy Award for Best Actress.

"Monster's Ball" was one of Diddy's first major film roles -- though he was an acting pro from the numerous music videos he appeared in during the mid-to-late 1990s.

Of course, Diddy's wearing a very different outfit behind bars these days while locked up during his federal sex trafficking and racketeering trial.

His ex-girlfriend, Cassie Ventura, testified against him earlier this month in the trial ... and, his former assistant -- going by the pseudonym "Mia" on the stand -- gave her testimony this week.

BTW ... if you'd rather stay away from Diddy memorabilia, GWS Auctions has a ton more items to sell -- including the mic stand used in the movie "Pulp Fiction" before John Travolta and Uma Thurman's iconic dance scene.