The 14-year-old was walking his dog through Cross Flats Park, on Beeston Road, when he was approached by an older youth and threatened with a knife.
The boy's dog bit the suspect who then stepped back.
But as he fled, he caught the youngster with the knife leaving him with a cut to his face.
The suspect - who was in his late teens to early 20s - was wearing a black beanie hat, a black neck scarf which covered the lower part of his face, a black Helly Hanson waterproof jacket, tracksuit bottoms and black gloves.
Police issued an appeal for witnesses today following the incident, which happened around 5pm on February 16.
Anyone who can help should call 101.