The singer, who is set to become a first-time father next year (16), paid a visit to popular Hollywood hangout Millions of Milkshakes with Danielle on Sunday night (06Dec15), and the couple appeared to be more than just friends.
A fan shared a photo of the pair on social media, while an eyewitness tells E! News Louis was "all over the girl" as they sat side-by-side in a booth awaiting their order of four decadent milkshakes.
"He was being playful and all lovey towards her, like couples do," the insider adds.
Louis and Danielle, who stars on U.S. vampire drama series The Originals, have yet to comment on the nature of their relationship.
They were also previously spotted out together at Los Angeles bar Bootsy Bellows last month (Nov15), when the actress' pal, Keelin Woodell, posted a picture of the smiling trio on Instagram.
Louis, 23, was recently rumoured to have rekindled his romance with Los Angeles-based stylist Briana Jungwirth, the mother of his unborn baby, after flying her out to London in September (15) for a One Direction gig at the city's O2 Arena, but it appears that is no longer the case.
The pop star made it clear he and Briana were not a couple when the baby news broke in July (15), but he has since admitted he is looking forward to spending time with the baby during the band's upcoming hiatus.
"I'm very excited, yeah," Louis said during an interview with Scottish TV outlet STV Glasgow in October (15). "It's gonna be really nice."