Around 4:30 p.m. Monday, a semi truck pulling an empty livestock trailer was traveling southbound on South Hoytsville Road.
Utah Highway Patrol Sergeant Cameron Roden said, according to witnesses, the 5-year-old boy darted out into the street.
The driver of the tractor trailer attempted to brake and steer around the child but the front right bumper hit the boy.
He was flown to an area hospital with critical injuries.
UHP said the driver of the tractor trailer is cooperating in ongoing investigation.