The 11-year-old was near a block of garages on Broom Nook in Belle Isle at about 5.45pm yesterday when the man approached him from behind and took hold of his shoulder.
The boy pulled away but the man grabbed him again before running off down a ginnel when a witness intervened.
The youngster – who was left distressed but unhurt – then returned home and raised the alarm.
West Yorkshire Police said yesterday’s events were at this stage not being linked to an incident in Beeston last Thursday when a girl was grabbed by a man in the street.
Det Insp Dave McDougal, of Leeds District CID, said: “The boy was not injured but was left distressed by what happened.
“We are carrying out enquiries to trace the suspect and would like to hear from anyone who witnessed the incident or who was in the area and saw someone fitting his description.
“We appreciate this incident will cause concern in the community and we are liaising with our colleagues in the local neighbourhood policing team who are increasing their patrols of the area to reassure people.”
The suspect is described as skinny and “quite tall” and was wearing grey tracksuit bottoms, gloves and a black jacket with the hood up.
Police say they are keen to trace the witness who intervened by shouting at the man.
He is described as white and in his 40s with grey hair and was wearing a white T-shirt and grey jeans.
The boy had been playing near the garages with a friend who went off to a local shop on his bike before the incident took place.
Anyone with information is asked to ring Leeds District CID via West Yorkshire Police’s 101 number, quoting log reference 13180203987, or Crimestoppers anonymously on 0800 555111.