Cardi B is proving she's a woman of her word ... posting an up-close look at her new butt crack piercing, after some fans doubted its existence.

Check it out ... the rapper posted on her X account Monday, where she declared to her more than 36 million followers that she had punctured her intergluteal cleft with some new "Drip" ... or as she eloquently put it, she had her "butt crack pierced."

But ... fans were quick to voice doubt about the bold addition to her already extensive list of tattoos and piercings on her body.

Rather than ignore the chatter, Cardi followed up with an upload of her bum's split ... which showed 2 diamond studs situated at the top of her bottom.

She added ... "Do I lie?"

It didn't take long for Cardi's followers to sound off on her decision to share the short-range shot of her ass. Some applauded CB for "keeping it real," whereas others slammed her for having too much time on her hands.

Cardi -- who has several piercings, including in her ears, nose, lip, chest, belly button, and eyebrow -- underwent the new piercing 4 months after welcoming her third child with ex-husband, Offset.

The "WAP" artist has yet to reveal her baby daughter's name ... but here's hoping that's her next big overshare on X!!!