Lunden, 63, who co-hosted GMA from 1980 until 1997, opened up about her diagnosis to current co-host Robin Roberts, who of course has battled cancer herself in recent years.
Lunden's cancer, which she called "the aggressive kind," didn't show up on a mammogram but was discovered during an ultrasound. She is beginning treatments shortly, including chemotherapy.
Lunden said she is optimistic about her prognosis, and urged women everywhere to get their annual checkups.
"Thank you to @robinRoberts and my @gma family for all your support this morning," Lunden wrote on Twitter after her interview.
On her blog, Lunden wrote: "I have decided to talk about my breast cancer because since the moment I took the job at Good Morning America I have lived my life sharing my joys and my disappointments with all of you: my pregnancies, my relationships, my career… I have shared my journey. So it certainly didn’t feel right keeping this part of my journey a secret."
Several high-profile journalists have had cancer scares lately. In addition to Roberts, who has battled breast cancer and myelodysplastic syndrome, GMA news anchor Amy Robach recently completed chemotherapy for breast cancer.