Authorities say the officer was putting air in a tire on his car at about 3:40 a.m. when a 19-year old man jumped in the driver's seat and tried to take the car.
The officer, a 23 year veteran, attempted to stop the suspect.
According to the Essex County Prosecutor, the suspect appeared to be reaching for a weapon, causing the officer to fire his service weapon one time.
The suspect then fled in the officer's vehicle.
A short time later, a maN suffering from a gunshot wound walked into the emergency room at University Hospital and was identified as the carjacking suspect.
The suspect, 19-year old Marcus Brant of Newark, is hospitalized in stable condiion.
Brent has been charged with carjacking, aggravated assault, conspiracy and weapons offenses, and is currently under police guard at University Hospital.
The off-duty officer was not injured during the incident. The officer's name is not being released at this time for security reasons, according to authorities, who say the investigation is continuing and additional arrests are possible.