An exterior rearview mirror system for a vehicle includes an exterior rearview mirror assembly that is mountable at an exterior side of a vehicle and has an inboard portion that is viewable by a driver of the vehicle when the mirror assembly is mounted at the exterior side of the vehicle. A blind spot indicator is disposed at the inboard portion of the mirror casing of the mirror assembly. The blind spot indicator comprises at least one illumination source for indicating to the driver a detected presence of an object alongside of and/or rearward of the vehicle. The indicator may comprise a unitary indicator module that is mountable at the inboard portion of the mirror assembly. The indicator module may include an illumination source and circuitry and may be connectable to an electrical connector.