The "Veep" star broke the news on Instagram.
"1 in 8 women get breast cancer. Today I'm the one. The good news is that I have the most glorious group of supportive and caring family and friends, and fantastic insurance through my union.
"The bad news is that not all women are so lucky, so let's fight all cancers and make universal health care a reality," Louis-Dreyfus wrote in her announcement.
"Just when you thought...," she captioned the post.
Louis-Dreyfus most recently made history at the Emmy Awards this month, winning her sixth trophy in a row for her work on HBO's "Veep" and tying Cloris Leachman's record eight Emmy wins by a singular performer.