Shawn Mendes
During her first night at the Rose Bowl in Pasadena, Calif., Taylor Swift surprised fans with the "Stitches" singer. The two performed "There's Nothing Holdin' Me Back" together.
Selena Gomez
The pop music superstar joined Swift for a duet of "Hands to Myself" during her second Rose Bowl concert. Swift wrote on her Instagram, "To the person I could call at any time of day, who has been there no matter what... you absolutely KILLED IT tonight and everyone was so excited to see you. I love you, and 60,000 people at the Rose Bowl loved you too."
Troye Sivan
The "My My My!" singer performed with Swift soon before Gomez at the Rose Bowl.
Niall Horan
At Wembley Stadium in London, the former One Direction member stunned Swifties with his hit "Slow Hands."
Robbie Williams
Swift and the British singer reunited in London for a performance of the 1997 hit "Angels." Swift wrote on her Instagram, "I'll NEVER forget the sound of the crowd when Robbie Williams showed up to sing Angels, that was UNREAL!! Thank you to Robbie, that was such a spectacular moment and it meant so much to so many people, including me."
Hayley Kiyoko
Swift and Hayley Kiyoko performed "Curious" at Gillette Stadium in Foxborough, Mass.
Bryan Adams
During her Toronto tour stop, Swift brought the Canadian rocker on stage to perform his song, "Summer of 69" together. Swift told the audience the hit was "one of my favorite songs ever written."
Tim McGraw and Faith Hill
Getting back to her country roots! Swift invited the country power couple to sing her debut single,"Tim McGraw" in Nashville and the crowd loved it.
Sugarland
Taking the stage in Dallas, Sugarland and Swift performed their first-ever live performance of their collaboration, "Babe." Swift originally wrote the song for her album, Red, before offering it to Sugarland.
Maren Morris
The 28-year-old country singer returned to her hometown of Dallas, Texas to sing her hit song, "The Middle" with Swift and it was the perfect mix of country and pop on one stage.
Camila Cabello
On March 1, Swift announced the "Never Be the Same" singer as an opening act for her reputation tour. Throughout her tour, you can catch the duo singing "Shake It Off."
Charli XCX
The "Boom Clap" singer also joins Cabello and Swift during "Shake It Off."