Anika Rashaun -- who starred on hit reality shows like "The Challenge" and "The Real World" -- has died from breast cancer ... her friends have revealed.

The TV star-turned-dating coach died on Oct. 1 after being diagnosed with cancer last summer.

Cyndi Rich wrote on Facebook ... "Anika shined so bright. Being close to that light as one of her closest friends was a true blessing. She is a vibrant soul. I’ll love her and treasure the 16 years of memories forever."

Another friend Tiara Ramos wrote on Substack ... "Anika Rashaun Walker Rehn was part of my life longer than she wasn’t. She was my mirror: the good, the bad, the ugly, the really ugly. She wasn’t just my friend. She was my other half. If you knew me, you knew her."

Tiara also wrote that despite the fact her mother had breast cancer years earlier ... "she was never identified as high-risk or told by her doctors to prioritize mammograms."

She added ... "That was one of the many failures she experienced within the medical system during her time battling cancer."

Anika -- who previously appeared on "The Real World" 2016-2017 "Bad Blood" season and on "The Challenge: Invasion of the Champions" in 2017 -- was 34.

RIP