The pop icon wrote on Tumblr, "I’m writing to you with an update I wish I wasn’t giving you, but it’s important and I’m used to sharing important events in my life with you ... This is something my family and I thought you should know about now."
Swift, 25, said her mother felt perfectly fine, but, "The results came in, and I’m saddened to tell you that my mom has been diagnosed with cancer."
The "Shake It Off" singer and her family will keep the details of the diagnosis private, but admitted her mother wanted all Swift's fans to know.
"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 added. "She wanted you to know why she may not be at as many shows this tour. She’s got an important battle to fight."
Swift ended the post by thanking her fans "for caring about my family so much."
"I hope and pray that you never get news like this," she wrote.