'My 600-Lb. Life' Star Dolly Martinez's Cause of Death Revealed

Dolly Martinez, who appeared on "My 600-Lb. Life," recently died ... and TMZ has learned more about her tragic passing.

Staci Thurman, Dolly’s mom, tells TMZ … the 30-year-old died of congestive heart failure at a Fort Worth hospital.

Dolly had been sick for a while with medical issues ... and we're told she went to the hospital on March 29 with fluid on her heart and lungs. She was put into a medically induced coma and placed on a ventilator the next day -- but never recovered.

Staci told us Dolly was surrounded by friends and family when she passed, which included her half-sister, Lindsey Cooper, who was her best friend. It was Lindsey who broke the news of Dolly's death via a Facebook post on Saturday.

Staci made it clear ... Dolly's family loved her deeply, and they're all devastated.

Dolly appeared on Season 10 of the TLC show, where she opened up about her struggles with food addiction and mental health. At the start of her episode, she weighed 593 pounds, relied on oxygen and needed assistance with daily tasks.

She later moved from Fort Worth to Houston to be closer to her doctor in hopes of qualifying for weight loss surgery. While she lost around 40 pounds, she was not approved for the procedure during her time on the show.

Dolly was 30.