The boy and his nine-year-old brother were holding hands with their grandmother as they walked down Bridge Street towards Ross Street in Glebe around 4. 20pm. when a man approached the trio and punched the boy in the face as he passed.
When the boy fell to the ground, the guy continued walking east of Bridge Road.
The grandmother helped the boy, who had suffered bruises on his chin, and his brother at home before contacting the police to record a report.
Officials say the guy has a dark skin, is about cm tall and is thin, has a long face with a dark beard and short, dark hair.
At the time of the punch, he wore a dark hoodie, long dark trousers and a trot.
The police are nice with data or photographs from the incident dashboard camera, to touch the police station of Leichardt or Crime Stoppers to 1800333000.