The "Shake It Off" singer was joined by friend and "Law & Order: Special Victims Unit" actress Mariska Hargitay on the stage's runway during a performance of her hit single "Style", reported Digital Spy.
Swift tweeted a link to a fan-filmed Instagram video of the moment the trio struck a pose on the catwalk after the show, writing: "What even happened tonight in Philly."
Earlier this month, Swift was joined on stage in Pittsburgh by Little Big Town to perform "Pontoon", while last month saw Imagine Dragons frontman Dan Reynolds surprise the audience in Detroit by singing a duet of "Radioactive" with the star.