The student was walking along Abergele road at 3pm last Friday when he was pushed from behind near the junction with St Catherine's Drive, causing him to fall and hit his head on a boundary wall.
Police are now appealing for witnesses to come forward with information about the incident.
Colwyn Bay police officer Matt Jones said: “This was an unprovoked assault.
“The offender is described as white, between 5ft8’ and 5ft 9’ tall of stocky build and in his 20’s and wearing grey jogging bottoms and a purple t shirt.
"The offender then ran towards Old Colwyn town.”
Anyone who witnessed the assault or has any information relating to the incident should contact Colwyn Bay police station on 101 or contact Crimestoppers anonymously on 0800 555 111.”