Rosie O'Donnell just showed up at a New York City courthouse to begin fighting her custody battle in front of a judge.
Rosie wasn't talkative but showed resolve as she entered the building.
TMZ broke the story ... Rosie's wife, Michelle Rounds, claims Rosie indulges in alcohol and weed to the point she is not capable of taking care of their 2-year-old daughter Dakota ... it's a stark contrast to the various pics we've seen of Rosie and her daughter.
For her part, Rosie says Michelle is a liar whose end game is squeezing money out of her.
Michelle's lawyer will be in court, asking the judge to order Rosie to submit to random drug testing until the custody matter is resolved. Rosie is asking the judge to enforce the prenup -- which has a confidentiality clause -- and gag Michelle.