"I just became one of the 300,000 women per year who are diagnosed with breast cancer," she said in a Facebook post.
Rep. Speier said it was discovered early, and she had a successful lumpectomy on Tuesday.
"I had a mammogram in June of 2023 and was scheduled for one in June this year. I almost didn't go because of summer activities. Thank goodness I didn't cancel."
She said she is being treated at UCSF.
"The lesson here is, please be diligent about your own screenings and don't skip them."
Speier served as the U.S. representative for California's 14th congressional district, serving in Congress from 2008 to 2023.
VIDEO: 40 years after Jonestown massacre: Why survivor Jackie Speier says we have to be vigilant about religious groups that operate in secret
40 years after Jonestown massacre: Why survivor Jackie Speier says we have to be vigilant about religious groups that operate in secret