The 24-year-old singer-songwriter and the 21-year-old actress were seen dancing and singing together at the concert in Las Vegas, sparking rumours that they are now officially dating.
The high-profile duo appeared to be having lots of fun together as they danced and laughed their way through the night in the VIP area, where fans were seen approaching them in the hope of getting a selfie.
Rumours of their supposed romance were first sparked late last year, when Niall shared a sweet birthday tribute to Hailee on Instagram.
At the time, he wrote: ''Happy birthday to the loveliest person on the planet and one of my best friends @haileesteinfeld. Have no idea what the hell is going on in this photo. Love ya hails (sic)''
And the actress gave further credence to the rumours during a subsequent red carpet appearance, when she was asked if she was dating the One Direction star.
She said: ''I feel so lucky that [my fans] care enough about me to know what I'm doing every second of every day, but I think that sometimes it's hard for people in general to realise that there are boundaries to a person's life and personal life.''
Hailee subsequently added: ''I do love that I have the opportunity to share with them what I'm comfortable with and they're there to listen and support me.''
Niall has been linked to a host of famous women over the years, including Ellie Goulding and Selena Gomez.
But in October 2017, he confirmed he was still single during a chat about his love life.
Niall confessed: ''I'm a very single man. You know, I'm 24 and travelling the world and that's the way it is.''
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