In a letter posted on ABCNews.com, Roberts said she found a lump during a self-exam after working on a piece about the late Joel Siegel, a show's film critic who died of cancer and was an advocate of early screening, ABC said Tuesday.
Roberts said she hoped her doctor would say it was nothing, but a biopsy "confirmed that the lump I'd found was indeed an early form of breast cancer.
"Hearing the doctor say those words out loud was surreal," Roberts wrote.
Roberts told viewers she was "blessed" to discover the cancer early and the prognosis is "so promising that my doctor expects me to be flying planes and hanging on to submarines .... in no time in the mornings to come."
She said she was grateful for the support of her family, friends and the "GMA" team.
"Now I join the ranks of millions of Americans who are fighting this same battle each and every day," she said. "I appreciate your courage and your example. Thank you for showing me the way."