Shawn Mendes joins Niall Horan's tour for surprise performance: Watch here

Shawn Mendes recently made a surprise appearance during Niall Horan's world tour.

The "Wonder" singer joined Horan onstage at his London show on March 1 and the two sang Mendes' 2016 hit song "Treat You Better."

Horan shared a video featuring the moment he affectionally introduced Mendes during the show, calling him his "little big brother."

Mendes then walked onstage, drawing huge cheers from the crowd.

Following the duet, the two embraced onstage and Horan told the crowd, "That's my boy!"

Horan captioned a clip of their performance, "Night Eight London. Thank you to the wonderful @shawnmendes for joining me on stage last night."

Horan previously teased special guests would be joining the tour while speaking about it with "Good Morning America."

"I've been putting this show together since I was about 10," he said. "The set list will be a combination of all the albums that I've made, some little bits of hidden surprises here and there, and maybe some special guests along the way I'd imagine...."

Mendes hasn't performed much since he officially cancelled his "Wonder" world tour in 2022 to focus on his mental health.

In an interview with WSJ. Magazine last year, Mendes described the process of stepping away from the tour as "very difficult" and said he was doing "a lot of doing therapy, a lot of trying to understand how I was feeling and what was making me feel that way."

He also said that his "last year-and-a-half has been the most eye-opening and growing and beautiful and just healing process of my life."

Apart from a few public appearances -- and a surprise performance with Ed Sheeran at Sheeran's show in Toronto in June -- the singer has maintained a relatively low profile and it's unclear if he will return to touring.