Donald Trump's mother is no orangutan-banger ... so says the Donald himself in a new, actually-filed lawsuit obtained by TMZ. Seriously, Trump actually filed this.

The Donald just sued Bill Maher for $5 MILLION ... claiming the HBO talk show host is skipping out on his promise to donate $5 mil to charity if Trump could prove he isn't the spawn of his mother having sex with an orangutan.

According to Trump, Maher made the offer during an appearance on "Leno" ... after Trump had made a $5 million challenge to President Obama regarding POTUS' birth certificate.

In his suit, filed today in L.A. County Superior Court, Trump says he formally responded to Maher's challenge with a copy of his birth certificate, proving he is, in fact, the "son of Fred Trump" ... who is not a member of the Great Ape family.

Trump claims 5 "worthy charities" have now been deprived of millions of dollars in donations ... and it's all Maher's fault.