A foldable display device is provided. The foldable display device may comprise a lower module, a display module on the lower module, a functional module on the display module, and a window module on the functional module. The display module may include a main region having a display area. The display module may include a lower inorganic encapsulation structure and an upper inorganic encapsulation structure directly contacting each other to form an inorganic-inorganic contact closed loop that substantially surrounds the display area in a plan view. The upper inorganic encapsulation structure may include at least three layers. Both of the upper inorganic encapsulation structure and the window module may be flexible.