The 53-year-old British actor is best known for playing Dawn Tinsley in the UK sitcom.
In a post, she said was diagnosed a year-and-a-half ago and the cancer has "metastasised" to her bones including her spine, right hip and ribs, and it is "too late" for chemotherapy.
"As of now, I'm trying to live out whatever might be left of my life in as fun a way as I possibly can," she said.
In her post, Davis also shared: "I always like to find the teachable moments in anything negative that happens. And cancer has not disappointed in that regard; there's a lot I have learned from it, and I'm grateful for that.
"As some of you will know, the pain can really be something else. Standing and walking for too long a time can be hard, and I need to use a wheelchair sometimes (so if you see me toddling around in one, feel free to give me a push!)"
Davis said one of the most important things that has helped her through the cancer is humour, asking her friends and family to poke fun at the actress "as much as possible" and above all to not treat her like a "sick person".
According to Cancer Research UK, stage four breast cancer is a cancer that starts in the breast tissue and that has spread to another part of the body (metastasised) making it incurable – however, treatment can control the cancer for some years.
Davis will continue working for animal rights, adding that she would also like to keep working as an actress - a job which is one of the "biggest joys" of her life.
Davis rose to fame playing receptionist Dawn in The Office, which follows the lives of office workers at Slough paper merchant Wernham Hogg with co-creator Ricky Gervais at the helm as office manager David Brent alongside stars such Martin Freeman - who played bored salesman Tim Canterbury.
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