Kim Zolciak doesn't need to say anything to make it clear she's not happy that her 15-year-old son is still sitting in jail ... because her silence here speaks volumes.

The 'RHOA' alum remained tight-lipped as a photog peppered her with questions about her son Kroy Biermann Jr.'s sexual battery arrest.

Kim was at Palm Beach International Airport -- excuse us, President Donald J. Trump International Airport -- Friday when a pap started grilling her ... and she let her actions do all the talking.

We broke the story ... KJ was arrested after a female classmate accused him of forcing her mouth on his penis and digitally penetrating her vagina in a gym changing room.

Kim vehemently denied the allegations in a statement to TMZ ... and she was in no mood to talk to this pap as she and her boyfriend, Kyle Mowitz, wheeled in a mountain of luggage.

Kroy Jr. remains in jail and is being held without bond ... and it's no surprise his mother isn't happy.