Jason Patric In A Bizarre Custody War

TMZ has learned the facts surfaced in a court hearing earlier this month -- Patric was with on-again-off-again girlfriend Danielle Schreiber for more than a decade. They broke up in early 2009, and he allegedly told her that he didn't have any money to give her but he would give her the next best thing -- his sperm, so she could have a baby.

We're told Jason made it clear ... he would only provide his sperm if she didn't breathe a word to anyone and never asked him for child support. She agreed.

Baby Gus was born in December, 2009. In 2011, Jason and Danielle got back together but it didn't last long. They broke up in May, 2012.

Apparently, while they were together, Jason, who never paid Danielle child support, took a liking to Gus, because he filed legal docs in family court asking a judge to award him 50/50 custody.

But there's a little known law which says if an unmarried man artificially inseminates an unmarried woman with the help of a physician, he's NOT legally the dad and has no custody rights.