A foldable display device including a lower module having a first side end disposed in a first side area of the foldable display device; a display module including: a main region including a display area and a first side end disposed in the first side area; a sub region disposed under the lower module, overlapping the main region, and having a first side end disposed in the first side area; and a bent region disposed between the first side ends of the main region and the sub region and having an outer surface between an upper surface of the main region and a lower surface of the sub region, and an inner surface between a lower surface of the main region and an upper surface of the sub region; and a functional module and a window module each having a first side end disposed in the first side area.