The child had returned home with his father who locked the car and went into his villa, believing the boy had already gotten off the vehicle.
The boy was found motionless in the car parked in front of their house in Al Ramsa area, police said.
He was taken to Al Qasimi Hospital in Al Khezami where he was pronounced dead, Khaleej Times reported.
The boy died due to suffocation and high temperature.
The Sharjah Police recently warned parents against leaving children locked in cars with the engines running.
More than five such cases were reported in Sharjah alone last year.