The incident happened just after 5:30 p.m. Saturday on Weldon Avenue near First and McKinley.
CHP officers say the 14-year-old, along with a 12-year-old girl, was approaching the intersection when the teen failed to stop at a stop sign, driving directly into the path of a Dodge Pickup truck.
The teen was ejected from the scooter and was taken to the hospital with major injuries.
The driver of the truck stayed on scene and was not injured.
Police say neither rider was wearing a helmet.