Police in Hull are appealing for witness to come forward following the incident at around 4pm yesterday at the junction of Lee Street and Holderness Road.
A Humberside Police spokesperson said: "The cyclist was riding along the footpath when he was approached by a boy aged between 14 and 16 who spoke to him and asked him for his bike.
"The cyclist didn’t respond and carried on riding when he was then punched to the face by the teenager, causing him to fall to the floor. The cyclist recovered and managed to ride off."
The 21-year-old victim suffered bruising to his face.
The spokesperson added: "The teenager suspected of the assault is described as being white, and of medium build with short, black hair.
"He was wearing a black Adidas jumper with three stripes along the arms and grey jogging bottoms."
Witnesses should call police on 101.