'Real Housewives of Atlanta' alum Kim Zolciak is firing back at allegations that her son Kroy Jr. sexually assaulted a classmate inside a gym ... claiming he even took a lie-detector test to prove it never happened.

The reality star tells TMZ ... "The allegations being made against my son are nothing short of reckless and deeply troubling. He categorically denies ever being involved with this woman or engaging in the conduct she has alleged."

Kim claims her son "voluntarily took a polygraph examination regarding these allegations and passed."

She also points to the police docs, which stated no male DNA was found on the alleged victim.

Kim claims the alleged victim made "similar allegations involving two other people, which raises serious concerns that deserve to be carefully examined." Her claim about the alleged victim has not been verified.

The reality star added that false accusations can have "devastating consequences," adding that "every legitimate allegation deserves to be taken seriously and investigated based on the facts and evidence.”

She said she was confident her son would get off if the case is heard in court.