The 56-year-old American actress revealed the diagnosis to her 750,000 Twitter followers, posting a note that read: "One in eight women get breast cancer. Today, I'm the one."
"The good news is that I have the most glorious group of supportive and caring friends, and fantastic insurance through my union," she continued.
"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, who has two children with actor Brad Hall, attached an image of the note to a tweet in which she wrote: "Just when you thought..."
The news comes less than two weeks after the "Veep" star won a sixth consecutive Emmy for comedy acting.
A native New Yorker of French stock, Louis-Dreyfus has been one of America's most popular and influential comedy actors since she found fame with cult sitcom "Seinfeld" in the 1990s.
On "Veep" she plays a somewhat bumbling vice president who later becomes the acting president, despite her hapless staff making political blunders along the way.
She has six consecutive best actress Emmys for "Veep" as well as three as an executive producer when it was awarded best comedy series.
She has won in the past for her roles on "Seinfeld" -- which also earned her a Golden Globe -- and "The New Adventures of Old Christine."
It has been a rollercoaster year or so for the actress, who tearfully dedicated her acting Emmy in 2016 to her father, who passed away two days earlier.
"I am so glad that he liked 'Veep,'" she said, her voice breaking down as she accepted the award. "Because his opinion was the one that really mattered."
"Veep" recently announced that the seventh season of the HBO show, due to premiere in 2018, will be its last.
