When police were called to the scene they were unable to locate the individual in question, but a man was later arrested at the Dorchester Hotel, where Justin is staying during his time in London.
A spokesperson for Scotland Yard said: "Police were called to Berkeley Square at approximately 9.45 last night. A man alleged he was assaulted.
"He suffered minor injuries. He did not require hospital treatment. Officers later arrested a 45-year-old man at a central London hotel on suspicion of common assault. He is being held at a central London police station. Enquiries are on-going."
Despite Justin and Mila being linked to one another romantically, the 30-year-old star recently admitted the pair's raunchy sex scenes in 'Friends with Benefits' were "awkward".
He said: "You sure didn't have to worry about being sexy. It's more awkward to shoot a naked scene where you're not going for laughs, those are more awkward to shoot. When your goal is to try to garner laughter, you just go for it.
"I think it's pretty obvious that I'm shameless, I'll put my body on the line for comedy, that's what I do. My butt is hilarious."