The singer was spotted taking Emma Ostilly out on Thursday (April 19) before kissing her goodnight on her doorstep at around 1.20am.
The 18-year-old also sneaked into the boyband's hotel following their gig in Auckland, New Zealand, reports the Sunday Mirror.
"They really seemed to have a connection and only had eyes for each other," an onlooker said. "They were enjoying themselves, laughing and joking together.
"They seemed very happy and relaxed and you could tell they have a history together. They went out again to a couple of bars in the city and then came back a few hours later.
"She ended up getting a lift home with Harry's driver in the early hours of the morning. Who knows if this will lead to anything. One thing is for certain, his female fans will be keeping a close eye on what is happening."
Styles first met Ostilly when she starred in One Direction's video for second single 'Gotta Be You', shot last autumn in her home city of New York.
The model, who is signed to the same agency as Beyoncé, is currently living in New Zealand.
Styles previously dated Xtra Factor host Caroline Flack and has since been linked to Irish photographer Sarah-Louise Colivet and Bachelorette star Jillian Harris.
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Watch a clip of One Direction on iCarly below: