The 28-year-old singer celebrated the couple’s engagement, which took place at Friday’s concert at Lincoln Financial Field in Philadelphia.
She shared the special moment in an Instagram post with the words: “They walk into the meet and greet and he says ‘We met 5 years ago at the Red Tour’ and then….. [sic] #thirdwheel“.
In three attached images, Swift looks shocked as a male fan gets down on one knee in the backstage Rep Room.
She is then delighted when his partner says yes and the couple embrace.
Following the big surprise, the groom-to-be, only known as Anthony, tweeted: “[Taylor] called me a baller and said that she is never surprised. She also said we were incredible and to go get married!!!! IS THIS REAL LIFE?!?!”
He added: “Thanks everyone for the love you are sending us! We did not expect this reaction at all! Making Taylor’s instagram is by far the coolest way to show off our perfect moment! Can’t wait to celebrate with round 2 tomorrow night! #reptourphilly“.
Anthony also revealed that his romance with partner Stephanie had blossomed at Swift’s Red Tour show back in 2013.
“We have spent countless hours listening to her music together. I can’t thank Taylor enough for bringing us closer,” he said.
Swift embarked on a romance with British actor Joe Alwyn, 27, nearly a year ago but the pair have kept their relationship out of the limelight.
She has returned in full force since the release of her 2017 album Reputation, which features hit songs Ready For It? and Gorgeous, and her subsequent world tour.