9/12 6:31 AM PT -- We spoke with the person who posted the fake photos to their page ... and, she says, "I’m sorry this was a fake page we just wanted followers! We’re minors!"

The baby girl Dave Grohl fathered outside his marriage is NOT the little tyke who is featured in a viral social media post claiming the Foo Fighter frontman is her pops.

Dave's rep tells TMZ ... the IG post claiming Dave's the father to a baby girl named Roxie Junie Grohl is fake news.

The post is spreading like wildfire on social media, after first being shared Wednesday afternoon by an Instagram account for a user named @valeriegreyson.

In the post Dave's camp says is fake, the user shares an image of a baby in a car seat holding a woman's finger. The caption says, in part, "my sweet Roxie Junie Grohl, being your mama has already been the most incredible experience of my life."

The post falsely claims ... "Your daddy, was by my side through the entire pregnancy, making sure we were both taken care of, and I'm so grateful for his love and support" and "every time I look at you, I see so much of your dad in your face ... you are truly a Grohl through and through."

Dave announced Tuesday he recently became a father to a baby daughter outside of his marriage to Jordyn Blum ... but the mother and child's name and identity remain a secret.