Mendes confirmed dating rumors, saying he and Baldwin were more than friends, but was reluctant to label his past with the 22-year-old model.
"I don't even wanna put a title on it. I think it was more of a zone of limbo," he told the magazine.
Mendes and Baldwin appeared to go public with their romance at the Met Gala in May. Baldwin got engaged to Bieber the next month and unfollowed Mendes on Instagram.
"I get it, you know," Mendes said. "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," he added. "I think I'm an idiot to not, you know... But you can't control your heart."
Bieber confirmed he and Baldwin quietly married in an Instagram post Friday.
"Happy late thanksgiving to everyone. First thanksgiving as a married man, first time hosting thanksgiving. First time both sides of the family has come together," the singer wrote.
Mendes told Rolling Stone he feels pressure to date, in part because of longstanding rumors he's gay, but rarely goes on dates. He released his third, self-titled album in May.