A vehicular exterior rearview mirror assembly includes a mirror reflective element, a mirror back plate, and a blind zone indication module having a light source that is electrically powered to emit light. When the light source of the blind zone indication module is electrically powered, light emitted by the light source passing through (i) a light-transmitting portion of a housing of the blind zone indication module and (ii) a light-transmitting aperture of the mirror back plate illuminates an icon that is viewable to a driver of the vehicle who views the mirror reflective element. Illumination of the icon is responsive to a blind spot detection system of the vehicle detecting presence of another vehicle that is traveling in the same direction and in a lane adjacent to a lane that the equipped vehicle is traveling in and that is approaching or is overtaking the equipped vehicle.