EXCLUSIVE

Famed cult leader Charles Manson is getting his day in court ... kind of.

Manson's reputed biological son Matthew Roberts will be participating in a performance art piece titled "The Retrial of Charles Manson" -- a mock trial at L.A.'s Vector Gallery.

Vector owner JJ Brine tells us he and friend Matthew Roberts created the retrial to put a spotlight on what they consider gross unfairness -- making Manson the symbol of evil when he didn't kill anyone.

Roberts will play the role of his father and serve as his own attorney ... something Manson wanted to do in the real trial but was denied.

The audience will serve as the jury and pass judgment. If convicted, the judge will impose a sentence.

FYI ... although Manson didn't physically kill anyone, it's more than clear he ordered the murders.