An eyewitness tells ET that the pair had a healthy date around lunchtime on Sunday at Hu Kitchen, a paleo-inspired eatery in Union Square.
"The two of them sat upstairs on the mezzanine level to eat, pretty secluded from the rest of the restaurant," the eyewitness says. "They looked very happy."
"Their interactions were flirty," the eyewitness adds. "They didn't explicitly hold hands or kiss that we saw, but they were definitely flirty."
Bieber, 24, ordered a freshly pressed green juice, the eyewitness tells us, while Baldwin, 21, was seen leaving the establishment with her juice in hand.
As for their interactions with fans inside the eatery, the eyewitness says Bieber and Baldwin "were great!" and more than happy to take pictures.
One of those lucky fans was Kendall Rivera, who shared the selfie she and her pals snapped with the lovebirds with ET. She tells us Bieber and Baldwin were "so nice," and "immediately wanted to have a conversation."
"They asked us how our Sunday was going, then I asked them if they had ever been here," she recalls. "Hailey said she had and that she likes to get the chicken. We then asked if we could take a selfie with them and they were super cool about it and very down!"
"Then, as they were leaving, they passed by our table and said, 'Hope you guys have a great rest of your day,'" she continues. "Overall, they were really, really nice!"
Bieber and Baldwin -- who a source says are "casually dating" after kindling their romance in recent weeks -- seem to be having a blast in the Big Apple. Over the weekend, they were also spotted making out in a garden in Brooklyn, seemingly paying no mind to the paparazzi who caught them mid-lip lock.