Swift is currently touring Europe but returns stateside in July for a North American run. If Bieber's going to join her for a song, that would be time to do it -- assuming he can get back on her good side.
The two last shared the stage together at the Staples Center in Los Angeles on August 23, 2011, to perform Bieber's hit "Baby."
Watch that performance here:
See the evolution of the Biebs:
The Evolution of Justin Bieber
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Canadian pop megastar Justin Bieber (C) plays basketball with a young survivor of Super Typhoon Haiyan in Palo, Leyte province on December 10, 2013. Pop megastar Justin Bieber sang to and played basketball with young survivors of the Philippines' deadliest typhoon on December 10 after he flew into the disaster zone to boost an international relief effort. AFP PHOTO (Photo credit should read STR/AFP/Getty Images)
LOS ANGELES, CA - JULY 27: The winner of Champ of Charity singer Justin Bieber (L) and singer Cody Simpson onstage at the 2014 Young Hollywood Awards brought to you by Samsung Galaxy at The Wiltern on July 27, 2014 in Los Angeles, California. The Young Hollywood Awards will air on Monday, July 28 8/7c on The CW. (Photo by Frederick M. Brown/Getty Images for Variety)
The "Baby" singer headed straight to the salon for a new makeover ... this time going blonde! (Instagram)
LOS ANGELES, CA - AUGUST 23: Justin Bieber attends a VMA Pre-Party hosted by Chris Brown & Pia Mia featuring Bera on August 23, 2014 in Los Angeles, California. (Photo by Araya Diaz/Getty Images)
NEW YORK, NY - JAN 17: General view of a Justin Bieber for Calvin Klein Billboard located on Houston Street on January 17, 2015 in New York City. (Photo by Tomas/IMAGES/Getty Images)
LAS VEGAS, NV - MARCH 15: Singer Justin Bieber arrives at Omnia Nightclub at Caesars Palace to celebrate his birthday on the nightclub's opening weekend on March 15, 2015 in Las Vegas, Nevada. (Photo by Gabe Ginsberg/Getty Images)
CANNES, FRANCE - NOVEMBER 07: Singer Justin Bieber attends the 17th NRJ Music Awards ceremony at Palais des Festivals on November 7, 2015 in Cannes, France. (Photo by Marc Piasecki/WireImage)
LOS ANGELES, CA - JANUARY 18: Justin Bieber and Corey Gamble are seen in Beverly Hills, Ca on January 18, 2016 in Los Angeles, California. (Photo by Light Brigade/Bauer-Griffin/GC Images)