A watertight apparatus of exterior component for a vehicle may include a coupling member coupling an exterior component and a vehicle body panel, and having a coupling groove formed on an outer circumferential surface of a part of the coupling member which passes through a vehicle body panel, and a watertight pad which is arranged to be in close contact with a bottom of the exterior component, a first side of which is coupled to a part of the vehicle body panel through which the coupling member passes and a second side of which is inserted into the coupling groove.