The Giuliana & Bill star made the announcement during an appearance on NBC's Today this morning.
Rancic, who is married to businessman Bill Rancic, told co-anchor Ann Curry that doctors discovered early stages of cancer when she went for her third round of IVF.
"Through my attempt to get pregnant for the third time through IVF we sadly found out I have early stages of breast cancer," she said.
"It's been a shock. I recently found this out and a lot of people have been asking, 'We saw in the season finale of the show that you went and got IVF so what happened? Are you pregnant?' But sadly we've had to put that off because of the news."
Rancic, 36, explained that her doctor had told her to get a mammogram before beginning her IVF treatment, and admitted that she had initially been reluctant.
"I never in my wildest dreams expected anything to be wrong so I went kicking and screaming to get a mammogram," she said. "They called me the next day and said, 'We need you to come back, we see something'."
Rancic went on to say that she will be undergoing surgery in the coming week before beginning six and a half weeks of radiation.
The E! News host said that she chose to speak about her diagnosis publicly to encourage other women to schedule mammograms and put their health first.
"I think a lot of us think that we are invincible and women these days are busier than ever," she said. "We're multitasking and taking care of a million people and things a day but we have to start putting ourselves on the to-do list.
"I want women out there to know that if you can find it early you will be okay. I will be okay because I found it early."
Rancic added that she hasn't given up her hopes of starting a family with her husband, saying that whenever they do have a child that baby will have "saved [her] life".
"I've always said there was some masterplan of why my IVFs didn't work and why I didn't get pregnant. Now I truly believe God is looking out for me. Had I gotten pregnant a few years down the line I could have been a lot sicker."
Watch more of Giuliana Rancic on Today below:
In a statement, the Style Network said: "We are sending our thoughts, love and support to Giuliana, Bill and their families during this difficult time. Giuliana has always shown great strength and courage in sharing her challenges with health issues facing so many women.
"We are hopeful that with the early detection she will have a complete recovery and that her personal journey will continue to be an inspiration and education for women everywhere."
E! has also released a statement, which reads: "On behalf of her E! family, we are sending our love and support to Giuliana and applaud her for sharing her recent breast cancer diagnosis in the hope that it will encourage women everywhere to take necessary and preventative measures.
"Giuliana has chosen to talk about her breast cancer on E! News tonight prior to her lumpectomy later this week. We are grateful that thanks to early detection, Giuliana is expected to have a swift and complete recovery."