The star of "Veep" and "Seinfeld" posted word of her illness Thursday on social media. A spokeswoman for Louis-Dreyfus confirmed the posts were authentic.
On her Twitter account, the actress wrote that one in eight women get breast cancer.
"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," she tweeted.
Louis-Dreyfus' publicist said no further details about her condition would be provided at this time, adding that the actress "is incredibly grateful for the outpouring of support and well wishes."
Hundreds of messages from well-wishers quickly poured in on Twitter.
"Sending you love and light and prayers of support," actress and producer Roma Downey wrote.
"We love you," ''Veep" co-star Tony Hale tweeted.
Louis-Dreyfus won her sixth Emmy Award on Sept. 17 for her portrayal of former POTUS Selina Meyer in HBO'S "Veep."
"Our love and support go out to Julia and her family at this time. We have every confidence she will get through this with her usual tenacity and undaunted spirit, and look forward to her return to health and to HBO for the final season of VEEP," HBO said in a statement.
Louis-Dreyfus received the diagnosis the day after the Emmys aired, and it played no part in the previously announced decision to end the show after next season, HBO said.
HBO said on Sept. 6 that the comedy's producers decided to bring it to a close after season seven, with an air date yet to be announced.
"Veep" writers will continue working on scripts and production will be adjusted as needed, HBO said.
Louis-Dreyfus is a Northwestern University alumna. Her youngest son, Charlie Hall, plays for the Northwestern University basketball team.