The boy had been walking on his own through Witty’s Passage, Newbegin, Hornsea, at around 5pm on Monday, when he was approached from behind by a cyclist.
The cyclist pushed him to the ground and stole his blue Samsung Galaxy A3 mobile phone.
The boy was not injured and has described the suspect as a tanned man with a stubbly beard and no front teeth. He was wearing a dark grey flat cap, a light grey, long sleeved jumper and blue denim knee length shorts. He was said to be riding a purple, white and blue mountain bike.
Anyone with information can call Humberside Police on 101 quoting crime reference number 16/65467/18.