Liz Wilcox -- who stars in season 46 of "Survivor," which is underway right now -- announced the sad news on X Friday ... saying she met Sonja over Christmas and shared a photo of the two talking on FaceTime on her account.

She writes, "Today, the legend herself Sonja Christopher of Season One passed away. I had the pleasure of meeting her on Christmas. She had so much spunk + love for Survivor and what the show brought to her life. I hope you’re singing + playing your heart out somewhere beautiful, Sonja."

On behalf of the family, Liz says people shouldn't send flowers ... but, instead donations to Mt. Diablo Unitarian Universalist Church in Walnut Creek, CA, the Cancer Support Community in SF, or the Sjogren’s Foundation. She did not provide a cause of death.

Sonja competed on "Survivor: Borneo" in 2000 and, despite getting along well with her teammates, got ousted after several missteps led to her team losing that first week. She received the boot because of it, making her the first person ever to be voted off the island.

In a sense, her appearance on the series was historic ... and many still remember her.

Christopher's known for playing her ukulele on the show -- and, she seemed to remember her time fondly on "Survivor" despite her early exit ... telling EW in 2020 the whole experience gave her a platform to raise money for causes she believed in.

Sonja was 87.