ENTERTAINMENT NEWS - "1 in 8 women get breast cancer," she wrote. "Today I'm the one."
"The good news", she said, was that she had a "glorious group" of family and friends supporting her and "fantastic" insurance through her union.
The actress, 56, has also featured in Saturday Night Live, The New Adventures of Old Christine and Seinfeld.
Earlier this month, Louis-Dreyfus picked up a record-breaking sixth Emmy Award for Outstanding Lead Actress in a row for her role in HBO series Veep.
In Veep, her character begins as an ineffectual US vice-president whose attempts to expand her role are continually thwarted.
Veep star 'would relish' drama roles
Louis-Dreyfus' announcement was met with an outpouring of support from other actors and celebrities.
Her Veep co-star Tony Hale retweeted the announcement, appending the message: "We love this woman."