According to police, the boy was sat on a wall in Basin Road near Chichester Police Station at around 11.15am on Thursday (June 16) when the incident happened.
Police say a boy struck the victim with a blade, believed to be wrapped in tape. He then ran off in a westerly direction.
Afterwards, the victim went to school where his injuries were treated, his parents were called, and the attack was reported to police.
A spokesperson for Sussex Police called it an ‘unprovoked attack’.
A boy has been arrested and released on bail until July 11, police say.
If you have any information please contact police online, or call 101, quoting serial 673 of 16/06.