The 56-year-old, who is most famous for her 1999 novel Chocolat, confirmed on her official Twitter account today that she has been diagnosed with the disease.
Harris said: "Today in #ThisWritingLife: In which Joanne learns she has breast cancer. Oh, 2020. You are really spoiling us.
"It is a primary cancer (which makes it sound like a cute toddler), so should respond to surgery and radiotherapy. But, yeah. It's still scary, and it sucks. So send me your pictures of otters, cats and recommendations for box sets.
"Thank you so much, all of you, for all your pics and good wishes. I can't possibly answer so many, but know that I see and appreciate you all very much. Now, tea, and maybe a rewatch of something fun on Netflix."
The writer was born in Barnsley and attended Wakefield Girls' High School before winning a place at Cambridge. She returned to Yorkshire to train as a teacher at the University of Sheffield, before spending 15 years teaching modern languages at Leeds Grammar School - Harris's mother is French and she is a fluent speaker.
Her bestselling novel Chocolat was published while she was still teaching, and her later works include Gentlemen and Players, which is set in a fictional boys' school that was based on her experiences at Leeds Grammar.
She is married to Kevin Harris, with whom she has a daughter, Anoushka, who works in publishing. She lives in Almondbury, near Huddersfield, where she writes from her garden shed.