Adrian Peterson -- NFL Star Has ANOTHER Secret Child

Adrian Peterson fathered a 5th child -- his 2nd secret child -- and the mother was a waitress at a popular Minnesota nightclub, TMZ has learned.

We've confirmed ... the child is a 3-month-old little girl who's living with her mom in Minnesota.

Sources close to the situation tell us the child's last name on her birth certificate is listed as "Peterson."

On the day Peterson's 2-year-old son Ty passed away, the mother of AP's 5th child posted an emotional message on her Facebook page about the situation.

"Today has been a long day finding out my [daughter's] brother passed away and knowing that she never even got to meet him."

Sources connected to the situation tell us ... the mother of AP's 5th child first met the NFL star while serving him as a VIP waitress at a place called Seven -- a steakhouse, sushi joint and ultralounge all wrapped in one.

It's unclear how involved Adrian has been in the child's life -- but the mother recently posted a photo showing the child decked out in Minnesota Vikings gear, so we're guessing they're cool.

Peterson has fathered 4 other children -- an 8-year-old daughter, a 4-year-old son, a 2-year-old son and another son, who died just months after AP learned he was the father.