The list of alleged Diddy victims involved in his sex trafficking trial is expanding … because we’ve learned the feds also have a witness they’re calling “Victim 5.”

Prosecutors buried the existence of a 5th alleged victim in legal documents last month, but “Victim 5” just came up in court Friday as Diddy’s trial proceeds.

Now, this person’s identity is still unknown, but that could soon change … because Judge Arun Subramanian ruled Friday “Victim 5” will NOT be granted anonymity in the trial.

According to the documents, obtained by TMZ, the feds appear to be introducing this potential witness as evidence Diddy committed an act of sexual misconduct in the past which he’s not being charged for in this criminal case.

Diddy’s legal team tried to keep this alleged victim’s story out of the trial — but Judge Subramanian is willing to allow at least some of the testimony.

As for the other 4 alleged victims … Cassie has long been established as “Victim 1,” and there are concerns “Victim 3” may not even show up in court to testify — despite a subpoena.

In court filings, prosecutors have said “Victim 2” is expected to deliver anonymous testimony about “financial losses, dependency, and social isolation she experienced during their relationship … all of which made her more vulnerable to his coercion.”