The cancer was discovered last week during a routine checkup, the 29-year-old said in a statement.
"My doctor was insistent that I have a mammogram even though it was not required. He saved my life!" she said.
"I appreciate all who are concerned, but please don't worry. Either way, I have no intention of letting this news get me down. I'm a fighter by nature and nothing will ever change that."
Representatives declined to discuss other details of the illness or her planned treatment.
Publicist Paul Freundlich said Anastacia is in dance rehearsals in New York and plans to finish filming a video for her latest single, "Love Is a Crime," featured on the soundtrack of the film "Chicago."
In May 2002, she performed on "VH1 Divas Las Vegas," sharing the stage with Celine Dion, Mary J. Blige, Cher and the Dixie Chicks at the MGM Grand.
The singer, who was born Anastacia Newkirk in Chicago and grew up in New York, got her start as a dancer on "Club MTV." Her albums include "Not That Kind" and "Freak of Nature."
When she was 13, Anastacia was diagnosed with Crohn's disease, a chronic inflammation of the digestive track. She has said she would never hide a large scar on her midriff, instead showing it off with formfitting cropped tops.