Humberside Police said the 12-year-old was robbed on Brandesburton Street, Hull, just after 8am on September 24.
A police spokesperson said: "The boy was allegedly pushed up against some metal railings and had his phone stolen from his pocket before the suspect ran off.
"The boy was left distressed but unhurt.
"The suspect has been described as white, around 18, 6ft 2in tall, slim and with a spotty complexion and a sallow face. He was wearing blue jeans, a black long-sleeved top and a black gilet."
Witnesses should call police on 101.