The 20-year-old singer was visiting the comedy quiz show when host James Corden convinced him to get a temporary tattoo of his face, on his bottom.
"We made a thing on the show that he's the only member of the band without a tattoo," Corde explained. "I said to him we've got this idea of putting a tattoo of my face on your body. Niall went, 'You should put it on my a**.'"
And they did.
"We tattooed it on to his left buttock - it took him three weeks to get rid of it. He sent me updates every week, with a photo of my fading face on his a** saying, 'It's still not gone yet'," the host added.
The episode, which was filmed earlier this year, will air on Sky1 on August 23. Horan reminisced of the incident with a post he wrote on Twitter on Monday.
What ya think of the pic of my arse and @jkcordens face?— Niall Horan (@NiallOfficial) August 12, 2013
Horan is the only member of "One Direction" who has yet to ink his body. Harry Styles, for one, has over 10 tattoos on his body.