The incident happened sometime between 2.55pm and 3.15pm on Sunday, 9 May at the junction of St Marys Road and Evans Street.
A 17-year-old boy was standing next to his bicycle on the pavement of Chapel Road, waiting to cross the road, when a man approached him and punched him in the face, rendering him unconscious.
The victim sustained minor facial injuries and was taken to the hospital for treatment.
The man is described as being either black or Asian and of muscular build. He was walking a Neapolitan Mastiff dog on a lead and was with a woman pushing a pushchair. They were walking up Chapel Road in the direction of the city centre.
We know the incident happened some time ago now but if you recall seeing the incident or recognise the description of the man with the distinctive breed of dog, then please get in touch.
Perhaps you were driving in the area at the time and have dash-cam footage?
Anyone who witnessed anything or has any information that may help the police investigation should call us on 101, quoting 44210178084.
Alternatively, people can contact Crimestoppers anonymously on 0800 555 111 or online at https://crimestoppers-uk.org/.