Bieber took to the photo-sharing platform Wednesday to share a series of photos of Baldwin, whom he married at a New York City courthouse in September.
Mendes, 20, “liked” one of the photos Bieber posted, specifically the image that shows Baldwin, 22, in a car.
Fans apparently interpreted the “In My Love” singer’s action to be flirtatious, People magazine reported.
But the 25-year-old “Let Me Love You” singer responded to the accusations by commenting: “Their [sic] friends relax,” according to a fan account’s screenshot of the comment, which was later shared by popular Instagram account “Comments By Celebs.”
Mendes and Baldwin faced dating rumors in the past, especially after they were spotted holding hands while walking the red carpet together at the Met Gala last year.
Mendes opened up about their relationship in November, telling Rolling Stone at the time: “I don’t even wanna put a title on it. I think it was more of a zone of limbo.”
Following Baldwin’s engagement to Bieber roughly a month after the Met Gala, however, Mendes told the publication he was quick to send the model a “congrats” message.
“I texted Hailey, ‘Congratulations,’ and I really am happy for them. She’s still one of the [expletive] coolest people ever — she’s not just a beautiful person visually, but she’s one of the most beautiful hearts I’ve ever met.”