The pair, who have a 23-year age gap, were seen partying in Ghana over the New Year and were spotted at a Davido Adeleke concert on Sunday.
Fans saw them at the London O2 Arena enjoying the Nigerian singer’s performance from a VIP suite.
A source told The Sun that the 25-year-old singer is “head over heels” for Ms Campbell, 48.
He split from Cheryl, with whom he has a son, Bear, in July.
The Sun reported that Liam has “raved to friends” about his new love, but fear he “may end up heartbroken.”
They added: “He can’t believe his luck.”
Another source revealed: “Liam has been seeing Naomi for a while.
“He’s mad keen on her. He seems head over heels.
“They are definitely an item and have been seeing a lot of each other.”
Naomi is said to have introduced Liam to fashion giants as he aims to break the modelling world.
Eagle-eyed Instagram users have also spotted the new couple flirting in each other’s comments.
On one post, a mirror selfie posted by Payne, Campbell commented “Beautiful soul”, to which the singer responded: “Takes one to know one.”
The model then shared a “pre-show” selfie ahead of walking the Louis Vuitton runway at Paris Fashion Week, prompting Payne to return the favour, commenting: “Perfection in a person… don’t give me those eyes.”