Police are appealing for witnesses after a young international student was assaulted and robbed in central Melbourne last night.

The 15-year-old, a student from South Korea, was walking west along the Bourke Street mall, about 30 metres from Swanston Street, when he was approached by four men about 8.30pm.

The men asked the boy for the time and when he looked down at his phone he was punched in the face by one of the men. The man grabbed the boy's phone and all four men then fled east along Bourke Street towards Swanston Street.

The teenager suffered minor injuries. The four men have been described as white but police have not released an estimate of their ages.

Police have asked anyone with information about the incident to contact Crime Stoppers on 1800 333 000 or visit www.crimestoppers.com.au.