Prince Jackson Stands Up for Grandma in Court

12:53 PM PT -- The judge just ruled Katherine knew exactly what she was doing when she replaced her previous attorney with Joy Bass ... saying there is no question in his mind.

Prince Jackson looked like man on a mission ... all in the name of his grandmother, Katherine Jackson.

Michael's son appeared in a Santa Monica court Monday for a hearing regarding Katherine getting a new attorney. The new lawyer is Joy Bass, and she says she's now representing Katherine in all matters related to Michael's estate.

This, apparently, was news to the Katherine's current attorney ... and sources close to the family tell TMZ Prince is in court now to tell the judge they're disputing the new attorney's claim she's now speaking for Katherine.

We got Prince and TJ Jackson (Tito's son) on the way into court and they were tight-lipped.