The Oscar-winning actress, 84, revealed the news on Instagram, explaining that it is a "very treatable cancer" for which she feels "very lucky".
Fonda, a lifelong activist, used the announcement to raise the issue of how many Americans "don't have access to quality health care" due to insurance troubles as well as discussing the effects fossil fuels can have on people's health.
Fonda shared an image of herself on Instagram and wrote: "So, my dear friends, I have something personal I want to share. I've been diagnosed with non-Hodgkin's Lymphoma and have started chemo treatments.
"This is a very treatable cancer. 80 per cent of people survive, so I feel very lucky.
"I'm also lucky because I have health insurance and access to the best doctors and treatments. I realise, and it's painful, that I am privileged in this.
"I'm doing chemo for six months and am handling the treatments quite well, and, believe me, I will not let any of this interfere with my climate activism.
"The midterms are looming and they are beyond consequential, so you can count on me to be right there together with you as we grow our army of climate champions."
Over a glittering Hollywood career Fonda has won two Academy Awards, two BAFTAs and seven Golden Globes.
Her films include 1971's Klute, 2005's Monster Law alongside Jennifer Lopez, 1978's Coming Home and she recently starred with Lily Tomlin in the hit Netflix series Grace and Frankie.
She is also known as much for her activism as her film career, and earlier this year she launched a special campaign group, named the Jane Fonda Political Action Committee, which is aimed at defeating politicians who support the ongoing use of fossil fuels.
26°C