The singer - who's admitted to having a soft spot for the Disney star - this week congratulated the pretty 19-year-old on her new role as a US X Factor judge and couldn't resist making a flirty remark.
'Congrats @ddlovato ...look after @SimonCowell uncle simon for us,' he Tweeted.
Though Demi's yet to reply, she's named Niall as her 'celebrity crush' in the past.
And of course 18-year-old Niall's cheeky Tweet didn't go unnoticed by his keen-eyed followers.
'@NiallOfficial OMG OMG OMG OMG NEMI!!!!!!!! ASK HER OUT NIALL NOW,' wrote one.
We think they'd make a great couple.
VIDEO: One Direction's Harry Styles supports Cancer Research UK
VIDEO: One Direction coming soon
VIDEO: Teen Now meets One Direction part 1VIDEO: Teen Now meets One Direction part 2
VIDEO: One Direction at Capital FM Jingle Bell Ball part 2