In a post to her 1.9m Instagram followers, the 84-year-old said she has non-Hodgkin's lymphoma.
Fonda described it as "a very treatable cancer... so I feel very lucky".
The Hollywood icon used her post to highlight the state of healthcare in the US and said she will continue with her climate activism.
She wrote she felt "lucky because I have health insurance and access to the best doctors and treatments.
"I realize, and it's painful, that I am privileged in this.