Fat Joe -- Notorious B.I.G. Mystery Revealed ... He Banged Anthony Mason's GF

It's one of the biggest mysteries in the history of hip-hop ... Which Knicks player was robbed by the Notorious B.I.G. after he walked in on Biggie banging his GF?

According to Fat Joe it was Anthony Mason ... and Joe made this BOMBSHELL revelation while talking with the guys on ESPN's "Highly Questionable" Monday morning.

The song is legendary ... called "I Got A Story To Tell," it chronicles Biggie's one night stand with a Knicks player's GF that almost ended up deadly when the unnamed player arrived home early.

Biggie maintained the story was true ... but for years we didn't know who it was until Joe spilled the beans earlier today.

Check out the clip ...