Kroy "KJ" Biermann Jr. had videos on his phone showing him having sex with an underage relative ... according to prosecutors in his sexual assault case.

Kim Zolciak and Kroy Biermann's eldest son appeared in a Georgia court Friday as part of a bond hearing ... and Page Six reports the prosecution claims investigators found videos on KJ's cell phone which they say depict the 15-year-old having penetrative sex with a relative under 12 years old.

It's unclear how old KJ was when the alleged videos were taken.

Deputy District Attorney Caitlin McGillicuddy claims there were "multiple encounters" on video found on KJ's phone. She also claims KJ uses "predatory, derogatory" language toward the alleged victim in the videos.

McGillicuddy says KJ was previously charged with sexual battery of a victim under the age of 16 ... but that wasn't made public before this because the case was in juvenile court. She says he's been under court supervision for that charge since October 2025.

As we reported ... KJ was arrested last month after a female classmate accused him of sexually assaulting her in a gym changing room. He's facing 7 criminal charges in the case.

Kim, Kroy and KJ's older sister -- Brielle -- reportedly appeared in court on Friday to show their support for him.

KJ's lawyer, Jason Sheffield, has asked the judge to release him on $65K bond ... McGillicuddy has asked the judge not to release him -- or at least to set bond at the much higher $190K.

We've reached out to KJ's camp ... so far, no word back.