Brian Austin Green -- '90210' Co-Star Stole $200k from Me!

Brian Austin Green has declared war on a former "90210" co-star who allegedly screwed him out of $200,000 ... and the accused happens to be the mother of his 9-year-old kid.

B.A.G. has filed a lawsuit against Vanessa Marcil-Giovinazzo -- the chick who played Donna's troublemaking cousin on the show -- claiming he let her BORROW a ton of money back when they were dating ... but she NEVER paid him back.

In the lawsuit, filed in L.A. County Superior Court, Green claims he fired off check after check after check to Vanessa ... but insisted "the monies were not gifts" and she promised to pay him back.

Green claims in 2000 -- BEFORE he knocked up Vanessa -- he loaned her up to $50,000 at a time ... and in total, lent her more than $200,000.

In the docs, Green says each time he loaned her money, he and Vanessa agreed that she would pay back the money "promptly" whenever he asked for it. Green says he came calling in November 2011 ... but she refuses to fork over the dough.

Green is demanding all of his cash back -- plus additional unspecified damages.