Simone Biles Denounces Jeffrey Dahmer Halloween Costumes, 'We Ain't Having It'

Considering being Jeffrey Dahmer for Halloween? Simone Biles says you better think twice about the costume ... telling people it's NOT okay to wear the 'fit.

"im just gon go head and say it," the Olympic gold medalist announced on Twitter on Tuesday, "put the jeffrey dahmer costumes back in the closet. we ain't having it!!!!!!"

The killer-themed getup has been a popular one this month ... in wake of the new Netflix show which shined a light on the murderer's life.

Wire-rimmed glasses, vintage shirts and blonde wigs are being sold all over ... but Biles is making it clear, she's fully against it all.

Dahmer murdered 17 young men -- and was accused of targeting marginalized communities to find his victims. Biles says all of that should spell out a no-go on the October costume idea.

The gymnast isn't the only one feeling that way ... Shirley Hughes, the mother of one of Dahmer's victims, told TMZ earlier this month the costumes are hurtful. She's advocated for online ads for Dahmer props to be taken down.