Swift posted the news on Tumblr, saying, "This is something my family and I thought you should know about now."
The 25-year-old singer said she had asked her mother to get a check-up at the doctor's office as her Christmas gift.
"There were no red flags and she felt perfectly fine, but she did it just to get me and my brother off her case about it," she wrote. "The results came in, and I'm saddened to tell you that my mom has been diagnosed with cancer."
Swift did not reveal what type of cancer her mother has or what type of treatment she would be receiving.
She encouraged her fans to remind their parents to get checked up as well.
"She wanted you to know because your parents may be too busy juggling everything they've got going on to go to the doctor, and maybe you reminding them to go get checked for cancer could possibly lead to an early diagnosis and an easier battle," she wrote.
Swift ended her post with "Thank you for caring about my family so much that she would want me to share this information with you. I hope and pray that you never get news like this."