"Real Housewives of Atlanta" star Kim Zolciak has a serious problem with the truth ... because she's a pathological liar ... this according to her own father.
It's the latest shot in the nasty family feud between Kim and her parents ... that started when Kim's folks told a Georgia court that Kim was unfairly blocking them from visiting her children.
Kim had fired back in court docs of her own ... claiming her mom and dad are bad grandparents and disrespectful people.
But Kim's dad Joe Zolciak tells TMZ his daughter is full of crap -- "Kim is a pathological liar PERIOD."
"I will not tolerate her trying to make a mockery of my wife or our family any further. We are completely humiliated by all the lies she has put in the media."
In fact, Joe says Kim is an unstable parent and a bad role model -- "She has been married twice and has three different fathers of her four children. Let's face the facts."
A judge has yet to decide on whether Kim's grandparents should have visitation rights.