Knowles, 67, told Good Morning America Wednesday he went to the doctor two months ago when he kept noticing a dot of blood appear on his shirts.
The music executive said he had a mammogram and was diagnosed with breast cancer.
He told Michael Strahan during the interview he has the BRCA2 gene mutation which also predisposes him to melanoma, prostate cancer and pancreatic cancer.
"I have to be very much aware and conscious, and do all of the early detection. Constant mammograms, constant prostate exams, constant MRIs for the rest of my life," said Knowles.
Knowles said he's now urging men to be more aware of breast cancer and get tested.