Credit: AP Photo/Charles Sykes

Phenoms unite!

Taylor Swift had more than one very special guest join her onstage Tuesday night at a sold-out show at NYC's Madison Square Garden, her last stop on her worldwide "Speak Now" tour.

The country superstar, 21, invited fellow singer Selena Gomez for a duet -- their very first together!

As the crowd went absolutely wild, Swift and her "best friend" Gomez, 18, went pop for a live rendition of Gomez's hit "Who Says?"

Later on in the show, Rock and Roll Hall of Famer James Taylor graced the stage for another first-ever duet of his classic "Fire and Rain." Swift told the crowd that she was named after the legendary singer, 63. The elder Taylor stayed onstage to play guitar for Swift's hit "15"

When Gomez (Justin Bieber's love) later the stage for a bow, she'd changed out of her sparkly mini-dress -- and was wearing a Taylor Swift tour T-shirt!

Tell Us: Who would you like Taylor Swift to duet with next?