Diem Brown Dead -- MTV Star Dies from Cancer at 34

"Real World/Road Rules Challenge" star Diem Brown has died after a long battle with cancer.

Diem -- who found out she had ovarian cancer 3 weeks before her first appearance on MTV in 2006 -- was reportedly surrounded by family this week in a New York hospital as her condition worsened.

Back in August, the reality star collapsed on set while filming "The Challenge" ... then announced she had been diagnosed with stomach and colon cancer. It eventually spread to her liver and lymph nodes as well.

Diem founded MedGift ... a support registry for other people battling illness.

One of her most inspirational moments on MTV came when she took off a wig to reveal her hair loss due to chemotherapy.

Diem was 34.