The "Bachelorette" -- who starred on Season 17 of the show -- broke the news in an Instagram post Friday where she shared parts of her story alongside a list of resources for others who have experienced sexual assault and rape.

Thurston says she's not ready to dive into specific details, but she explains she called 911 to report the incident ... and, an officer took her to receive a sexual assault forensic exam.

Katie says she felt like a "vulnerable child" during the process ... and credits the nurse and advocate for helping her through the examination.

Thurston says the kit took four months to be processed ... during which time she consulted with a therapist and read more about how to navigate conversations with detectives as they looked into her claims.

KT says she handed over everything to investigators -- texts, phone call records, DNA, videos and her own body -- and, yet she says her quest for justice was stopped in some way. She does not clarify what she means by that.

The reality star says this isn't a loss for just her ... it's "yet another loss in a broken system. Change happens in failure and the justice system failed me. And continues to fail thousands every single day."

She ends her post with an uplifting message to fans ... telling them they are valued, strong -- and, most importantly, she believes them.

Katie ended up with Blake Moynes in her season of "The Bachelorette" though the two broke up in 2021. She then dated John Hershey who also appeared on her season before they split in 2022.