Swift visited 13-year-old Delaney Clements at her home in Grand Junction, Colorado, while en route to Tennessee.
She and the whole Clements family spent some quality time together, with the selfies to prove it.
"Merry Christmas Baby," Swift posted on her own Instagram.
"Literally speechless right now!!!!!" Clements posted on Instagram, along with a photo of her and the "Style" singer. "My new best friend (as I call her now) made a special stop from going home to Tennessee and surprised me and gave me the best present/ hugg ever!! Thank you !! I love you!! Happy holidays!!"
Taylor Swift's Open Letter Forces Apple to Change Its Tune
"So this just happened," mom Wendy Campbell posted on Facebook Saturday afternoon, along with a dozen photos from the visit. "Thank you Taylor Swift for the best Christmas present ever."
The story of Clements' meeting with the pop superstar actually begins with a hashtag (#delaneymeettaylor), which was circulated as Clements continued to fight against the neuroblastoma she was first diagnosed with at age 8.
On Dec. 8, her mom announced on Facebook that Clements would be returning home for hospice care, after the news that the cancer had spread to her brain – "something we had been able to avoid for so many years."
"We are so blessed to be surrounded with so many wonderful people who love and care for us and forever grateful for all the support we have received over the past five years," Campbell wrote.
"Our goal now is to keep Delaney as comfortable as possible and keep her spirits uplifted as she travels on a journey we fought so hard to avoid."
Still, Clements was undaunted. She posted on Instagram earlier this week, "When your best friend comes to basically live with you and take care of you once you get home and it's basically the best present ever!!"
And on Saturday, Clements got to meet her idol.
After all, as Clements explained on Instagram, she and Swift share a few things in common: "Living our Wildest Dreams and Shaking It Off in Style."