The 46-year-old joker was being interviewed by presenters Alex Jones and Gethin Jones, and waited until the end of their chat for the big reveal.
Paddy, who was wearing a denim shirt up top, wrapped up the interview by saying: "Anyhow, have a good 'un!"
He then stood up, bringing his crotch into frame and revealing his white underpants.
Well, @PaddyMcGuinness - that is one way to exit the show! :scream: :joy:#TheOneShow pic.twitter.com/KvTDafHVme
- BBC The One Show (@BBCTheOneShow) August 7, 2020
Gethin was the first to clock what was happening on the screen behind him, and immediately burst into laughter, along with other members of the crew.
It took Alex a few moments to realise Paddy's mischief, and she appeared understandably taken aback, stopping in her tracks for a few seconds while it sank in.
"Aww, see!" she said with a smirk, before managing to pull it together to sign off the show.
"We'll be back on Monday, have a good weekend," Alex added, before shaking her head with a laugh.
Many viewers were as shocked as the show's hosts, with one tweeting after the episode that he'd 'caused quite a stir'.
Another person said: "Made my night with a lovely sight WOW!! As if it wasn't hot enough lol!!!!"
Someone else wrote: "Only Paddy would get away with it."
Others commended the comedian for his 'brilliant' banter, saying he had 'great timing'.
"Awesome job, Paddy," another said, adding five crying with laughter emojis.
Sharing someone's screenshot of the moment, Paddy tweeted: "To be fair it's hot in our house."
It's probably no wonder Paddy's willing to show off his bod at the moment, having recently shed half a stone after piling on the pounds during lockdown (and joking that he could no longer see his penis).
He posted the news on his Instagram account, sharing a photo of his scales to prove he had lost the weight.
"Five days later, 7lbs lighter," the star wrote in the caption.
"Couple of short bike rides and no cake, happy with that. Only one way to celebrate, cake!"
Of course, losing 7lbs in just five days is a fairly big achievement, with many fans joking that he may have found a way to bend the truth slightly.
One said: "Did you find a sympathetic tile in the bathroom?"
Paddy replied: "I was holding onto the top of the shower."
Featured Image Credit: BBC