The Irish heartthrob confessed his feelings for Katy after she called him "cute" following her performance on The X Factor last night (20 October).
Katy, 28, complimented Niall after the show's host Dermot O'Leary asked the popstar if she would ever return to the judging panel in the near future.
Katy was a guest judge on The X Factor in 2010 and had the final vote of putting through Niall at the audition stages through to bootcamp.
The 'Roar' hitmaker told the studio audience: "Oh my goodness, I would love to give constructive criticism. I liked that I let Niall through at one point. He's very cute, he's in a little band called One Direction."
I love you @katyperry ! Thank you! Ps . Will u marry me?— Niall Horan (@NiallOfficial) October 21, 2013
Dermot also joked: "One fifth of One Direction, that's because of you!"
After hearing Katy's praise, Niall took to Twitter and jokingly said: "I love you @katyperry ! Thank you! Ps. Will u marry me?"
Katy, who has released her fourth studio album Prism in the UK today, is yet to publicly reply to Niall's proposal. Although, we reckon the pair will remain just good friends, seeing as Katy has rekindled her romance with musician John Mayer!
Katy and Niall are quite fond of each other though, and last year the pair sent fans into a frenzy as they embraced on stage at the VMAs. After Katy handed 1D a music award, she ended up locking lips with Niall!
Niall and Katy even continued to flirt with one another on Twitter after the smooch, with Katy saying: "I'll be your Mrs. Robinson..."
Watch Katy's video for 'Roar' below: