The teen, believed to be between 14 and 16 years old, was running across Louetta Road and away from a convenience store after he had taken an item without paying for it, according to the sheriff.
The store clerk said the boy grabbed some ice cream and ran out the door before he was struck moments later.
The teenager was hit by the truck and thrown at least 20 feet across the roadway. A deputy said he appeared to have a head injury. There was damage to the front of the truck as well, where the posted speed limit on the road is 45 mph.
The driver of the truck who struck the teenager stopped after the incident. Westbound lanes of the road were closed for two hours until the investigation was completed.
Deputies were seen picking up what appeared to be the boy's shoes and a baseball cap from the street.
The teenager was said to be conscious when emergency responders arrived and was taken to a hospital with serious injuries.
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Teen struck by vehicle in north Harris County