Actress Gabrielle Union is unleashing hell … and her lawyers … on a gossip website that printed a scandalous, anonymous letter about her — a letter Union claims is filled with terrible lies.

Gabrielle’s lawyers fired off an ultimatum to the site, demanding they remove the letter, print a retraction and send an apology letter to Gabrielle… or else.

The site claims they received “the letter” from a “nameless pro-athlete’s wife” — and decided to print it because “it seemed too real.”

But Gabrielle claims the allegations in the letter — mostly involving sexual exploits — are 100% BS … and the site should have been more responsible with their judgment before deciding to print.

A friend of the actress spoke about the matter.

“I want your readers to know that it’s all completely false,” the friend told me. ”Gabrielle will move forward with legal action against the writer and the site. All she asked for was a retraction and apology and to take it off their site, but they refused and now her goal is to make them pay! They straight-up published a manifesto of a crazy person and they had to know it wasn’t true.“

“I almost get to a point where you can say whatever you want to say about me, but print some facts about our community that can help somebody,” she said then. ”I don’t care because I combat lies. I really used to think that if you read it and you see it and it’s in a magazine, a newspaper or you see it on TV, then it has to be true. And it’s just not true! I don’t even know how to combat that except for do more good things for the people that I care about which is my own community.”

Now, the Omaha, Nebraska-bred actress has reached her boiling point and refuses to be a punching bag for arbitrary blogs any longer.

“If there was any truth to it, she would’ve just let it die,” her friend confessed, before adding: “But she’s a black actress who really can’t afford to sue but intends to go broke fighting this if that’s what it takes!”

Maybe folks will think twice before printing unsubstantiated stories next time.