Officers went to the 1000 block of Ralston Avenue on a report of a crash involving a bicyclist at around 9:40 a.m., according to Belmont police.
The teenager was riding east when the driver of a car parked at the curb pulled into traffic and struck him.
The boy, who was wearing a helmet at the time of the crash, was taken to a hospital with moderate injuries, police said.
Eastbound traffic on Ralston Avenue was diverted onto adjacent streets for about 30 minutes while officers investigated the crash.
The driver of the vehicle was not given a traffic citation.