Shawn Mendes Makes Surprise Appearance at Niall Horan Concert for 'Treat You Better' Duet: 'That's My Boy!'

Niall Horan and Shawn Mendes sure know how to put on a show.

While in London performing his The Show: Live on Tour concert, Horan welcomed his Mendes to the stage as a surprise guest, and the two played an acoustic version of Mendes’ 2016 song “Treat You Better.”

In videos shot by fans and shared to social media, Horan, 30, and Mendes, whom he jokingly called his "little big brother," shared a hug before they each sat on a stool with a guitar and played the 2016 hit at OVO Arena Wembley.

"By the way, I was just thinking, waste of time going to the gym," Horan quipped to the crowd, in reference to Mendes' impressive physique.

"If you're gonna wear a tank top, I'm gonna wear a tank top," Mendes, 25, responded.

Horan then said, "Let's do this thing, brother," before playing the track, and later proudly said, "That's my boy!" after Mendes left the stage, according to fan video.

The performance came just hours after Mendes was photographed in Paris attending the Loewe fall/winter 2024 show at Paris Fashion Week.

It also marked just the second performance for the star since he canceled his Wonder world tour in July 2022 in order to tend to his mental health.

The “Stitches” singer was previously a surprise guest during Ed Sheeran’s Toronto show in June, where the two sang “Lego House” and “There’s Nothin’ Holding Me Back.”

In an interview last year with WSJ. Magazine, Mendes said canceling his tour was something inspired by the time he spent in therapy trying to unpack all that he was dealing with.

"The process was very difficult. A lot of doing therapy, a lot of trying to understand how I was feeling and what was making me feel that way. And then doing the work to help myself and heal," he told the outlet. "And also leaning on people in my life to help a little bit."

In January, Mendes shared videos of him playing the piano in a studio to Instagram, and explained his frustrations with waiting for “inspiration to strike.”

“Eventually I remember how important the willingness to just show up is,” he wrote. “How showing up and trying even with nothing is kinda like sending a message to the sky saying ‘I’m here and willing and open’ and usually, that’s when my hands stumble across a special little chord or my voice a melody that I like.”

Horan, meanwhile, is currently in the midst of a lengthy string of dates in support of his third album The Show, which he released in June.

After kicking things off on Feb. 20 in Northern Ireland, Horan will take the tour stateside in May and will play on through the summer before wrapping things up in London in September.

In addition to Mendes, he welcomed Dermot Kennedy to the stage as a special guest at his Feb. 23 show in Dublin.