Oliva Munn says she was diagnosed with breast cancer and had a double mastectomy last year.
She made the announcement in an Instagram post on Wednesday.
Munn, 43, said she had four surgeries over 10 months after the initial diagnosis.
Munn wrote, "I hope by sharing this it will help others find comfort, inspiration and support on their own journey."
Her boyfriend John Mulaney responded to the post with admiration.
"Thank you for fighting so hard to be here for us. Malc and I adore you," he wrote.
Malcolm is the 2-year-old boy the couple shares.
In the post, Munn said she underwent genetic testing in Feb. 2023 to check for 90 different cancer genes, including BRCA, which is associated with breast cancer.
She said her test came back negative and a mammogram that winter was also clear.
But two months later, she was diagnosed with breast cancer.
"In the past ten months I have had four surgeries, so many days spent in bed I can't even count and have learned more about cancer, cancer treatment and hormones than I ever could have imagined," Munn wrote.
Just 30 days after that biopsy, Munn underwent a double mastectomy.
"I went from feeling completely fine one day, to waking up in a hospital bed after a 10-hour surgery the next," Munn wrote.
Munn said she's kept the diagnosis private and "tabled any emotions that I felt would interfere with my ability to stay clearheaded."
"I'm so thankful to my friends and family for loving me through this. I'm so thankful to John for the nights he spent researching what every option and medication meant and what side effects I could expect," she wrote.
"For being there before I went into each surgery and being there when I woke up, always placing framed photos of our little boy Malcolm so it would be the first thing I saw when I opened my eyes."
The pair attended the Academy Awards ceremony on Sunday in Los Angeles.
