Zach Galifianakis once stole Justin Bieber's underwear.

The 49-year-old actor has admitted he pinched a pair of underpants from the 'Sorry' hitmaker back in 2014, after the singer appeared on his online talk show 'Between Two Ferns'.

Zach slimed himself on camera during the episode and had to shower in Justin's trailer, when he realised he didn't bring any spare underwear and so stole a pair belonging to the 25-year-old pop star.

Recalling the event, the 'Baskets' star said: ''I had to change clothes ... I got into his trailer to get this stuff off of me. I didn't have any underwear so I see a pair of underwear in the corner. I needed a pair of underwear ... I put 'em on.''

But Zach regrets his actions, as he now understands it was ''wrong'' to have taken the underwear.

He added: ''Then I started thinking about it. [On] a lot of levels it's wrong.''

Despite knowing it was wrong to take Justin's clothes, the 'Hangover' star never returned the garment, and instead offered the bizarre piece of Justin Bieber memorabilia to his young nieces.

Speaking to David Letterman on his Netflix show 'My Next Guest Needs No Introduction', Zach explained: ''I took the underwear. Years pass, and I had taken the underwear with me ... because I have nieces.

''So, we walk up to my bedroom, my sister and my niece, and we go up, and I'm like, 'Hey, I've been saving these for you.' And the look on my niece's face ... She did not accept it. I tried - she was not into it at all.''