The couple were seen cuddling up courtside at a basketball game in London, which saw the Brooklyn Nets beat the Atlanta Hawks.
Liam, 20, and university student Sophia had even coordinated their outfits for the evening, with both of them dressing head to toe in black.
The singer confirmed his romance with Sophia, his childhood crush, at a press conference for the band's film in August, and soon after they made their first public appearance together at the afterparty for This Is Us.
Meanwhile, Liam offered an apology to his fans this week after he was pictured standing on a 360 foot high balcony ledge.
Referring to the image, which had been posted on Twitter but had since been removed, he said, "You may have seen a photo of me today taken on top of a building. I regret being there and having a photo taken.
"It was a stupid and irresponsible thing to do. I am sorry, and do not endorse any fans trying to repeat this, as it is extremely dangerous."