The 37-year-old made the announcement Monday on NBC's "Today" show.
"It was not an easy decision but it was the best decision for me," she said.
She plans to have the surgery next week and said she hopes to be recovered by New Year's Eve, when she and husband Bill Rancic are planning to be in Times Square.
Rancic said she received an "overwhelming outpouring of love, prayers and support" after announcing her breast cancer diagnosis in October.
"I want to make sure to thank everyone and give them an update for being so kind and loving and supportive," she said Monday.
E! expressed its support for Rancic in a statement Monday.
"Giuliana's strength continues to amaze us," the network said. "We admire her courage and are proud to stand by her side through every step on her path to full recovery."