The 'I'm Outta Love' singer has been forced to cancel her European tour, which was due to start in London on April 6, in order to battle the disease.
The 44-year-old said: "I feel so awful to be letting down all my amazing fans who were looking forward to 'It's A Man's World Tour'. It just breaks my heart to disappoint them."
A post on her official Facebook page told fans: "Now more than ever, she is determined to live by her motto "Don't ever let cancer get the 'Best of You'!"
"A born survivor, Anastacia has one goal and that is to make a full recovery with the support of her family, friends and everyone around her."
The singer was first diagnosed with breast cancer in 2003, but overcame the disease following successful surgery and radiotherapy.
She will continue to write her new album and hopes to reschedule the tour for a later date.
Watch the video for Anastacia's 'I'm Outta Love' below: