The 37-year-old made the announcement Monday on NBC's "Today" show.
Rancic says the cancer was discovered during a mammogram while she was undergoing a third round of in-vitro fertilization in an effort to get pregnant.
Rancic says she will have surgery this week, followed by six and one-half weeks of radiation therapy.
She is married to Bill Rancic, winner of the first season of "The Apprentice."
Rancic says she still wants to get pregnant because "this baby will have saved my life."