The boy band singer, 18, had just finished performing with his bandmates Niall Horan, Zayn Malik, Liam Payne and Louis Tomlinson at New York City's famed Madison Square Garden. "It was the best night of our lives!" Horan, 19, tweeted. "Thank you, thank you, thank you!"
The new couple joined the other One Direction members for karaoke at the Hudson Hotel, according to one Twitter user who snapped a picture of Swift, Styles and Horan performing the Backstreet Boys' 1998 hit "I Want It That Way." Just before 4:15 a.m., Swift and Styles were photographed holding hands outside the "Red" singer's hotel.
The young musicians -- who were spotted flirting backstage at The X Factor in mid-November -- were first photographed together in Manhattan's Central Park Dec. 2. They were joined by Styles' stylist, Lou Teasdale, her fiance, Tom Atkin, and their 1-year-old daughter, Lux.