The boy was riding his bicycle near Manor Road park, Scarborough, when he was involved in a collision with another cyclist.
Police said the cyclist then grabbed the youth and punched him in his mouth, splitting his lip.
The suspect is described as a white man, of teenage years with short blonde curly hair.
He was of slim build wearing a grey Adidas tracksuit top and bottoms, and riding a black mountain bike.
Anyone with information about the incident is asked to contact police on 101.