Over the weekend, Niall was romantically linked to the 20-year old brunette, with reports suggesting that the Irish singer was calling Green his girlfriend.
A mole also told The Mirror: "They're dating – it's early days but they're very cute together."
However, during an interview with Now magazine, Niall cleared up any speculation regarding his love life, telling the publication: "[I'm] definitely single.
"I'm not actually looking but if the right person came along, [I'd] definitely [want a girlfriend]."
He was also quizzed about his on-stage kiss with Katy Perry at the 2012 MTV VMAs in September, stating: "Well, walking up those stairs the first thing I saw was her with her lips out. I forgot about the award at that moment - I just went straight for her lips!"
He continued to gush: "Oh man - she's amazing! I think she feels a little bit of ownership over me because she was the one to put me through on X Factor when she was a guest judge.
"I don't mind - she isn't a bad person to be making comments on ya! Katy's super-hot. And she's really cool as well. She was the first person to say hi backstage. Our dressing room was next to hers and we were just chatting and stuff."