But the strong star said she wouldn't let it beat her.
"My will, my faith and my body have been
challenged," she said.
"But make no mistake, my heart is strong and my resolve to fight will never be broken.
"To all the people that have reached out to wish me well, I thank you from the bottom of my heart."
Anastacia also begged other women to have regular tests for breast cancer, and asked them to learn from her experience.
The star, who's sold more than five million albums, also had an operation to treat a bad bowel problem, called Crohn's Disease, when she was just 13 years old.