He's willing to pay up to $500 a pair.
The singer is the latest guest to feature on James Corden‘s Carpool Karaoke.
Speaking to Corden about moving into a house on his own, he admitted his mum comes round to clean and do laundry.
“You know Justin Bieber just wear a new pair of underpants every day?” Corden says.
“Wait, so he then just throws them in the garbage?” asks Mendes.
“I think he then sells them online…” jokes Corden.
“I’d buy them,” Mendes quips, raising a bottle of water to his mouth. “Wouldn’t you?”
When asked how much he’d pay for them, he jokes that he’d cap it at $500, but would need to know what Bieber had done in the pants.
The singer also spoke about his love for Harry Potter.
“On a scale of ten being crazed superfan and zero is, hate it, never seen it, where are you?” Corden asked Mendes.
“9.5,” Mendes responded. “I have a whole book at home that I wrote all the spells in and stuff.”
He then appeared dresses as Dumbledore.
- READ MORE: Shawn Mendes – ‘Shawn Mendes’ Review
Meanwhile, the singer recently opened up about his struggles with anxiety, and how it shaped the sound of his self-titled third album, which came out May 25.
The singer addresses his own battles with mental health on recent single ‘In My Blood’, and he told NME: “It’s all about honesty”.
Shawn Mendes is touring the UK in 2019 – buy tickets here.