Mendes, 22, and Bieber, 26, released a single and music video for the track Friday.
In the "Monster" video, Mendes and Bieber reflect on the downsides of celebrity while singing from atop a pedestal in a storm. The singers question how fame lifts them up and tears them down.
"What if I, what if I trip? / What if I, what if I fall? / Then am I the monster? / Just let me know / And what if I, what if I sin? / And what if I, what if I break? / Then am I the monster?" the pair sing.
In an interview with Zane Lowe for Apple Music this week, Mendes said he wrote "Monster" while "coming off the back of a couple big songs" and being "so driven" by ego but feeling like something was not right.
"This song has always resonated with me and it just never found its way out. And now all of a sudden I was listening to it one day, and I just started my relationship with Justin," Mendes said.
"It's about how society can put celebrities up on a pedestal and watch them fall and it seems to be this entertainment thing. And it's hard," he added. "And the more I listened to it and the deeper I get into it, this is not just about celebrities. This is the human experience."
"Monster" appears on Mendes' forthcoming fourth studio album, Wonder. Mendes said on The Tonight Show starring Jimmy Fallon in October that his girlfriend, singer Camila Cabello, supported him through the making of the album.
Mendes gives more insight into Wonder and his life in the Netflix documentary Shawn Mendes: In Wonder, slated for release Monday. Mendes shared a trailer for the film in October.