A 15-year-old Justin Bieber told a racist joke to friends ... knowing his own photog was in the room and rolling.
The video was shot backstage at a promotional event early in his career ... where Justin cracked this terrible joke ... "Why are black people afraid of chain saws?" His punchline, repeatedly saying, "Run N****r."
TMZ got this video 4 years ago but we decided not to post it ... in large part because he was 15 and immediately told his friends what he did was stupid. People connected with Bieber say one African American was present at the time he told the joke, although that really doesn't matter much.
The video was posted Sunday on a British pay site.
Sources connected with Bieber say he is "frustrated and sad" and now wants to address it publicly, which we're told he will do in the next day.