A foldable display device includes a flexible display module having an electroluminescent unit, a flexible functional module on the flexible display, the flexible functional module having at least one function selected from the group of polarization, a color filtering, color conversion, a touch sensing, and biometric information recognition, and a flexible window module on the flexible functional module. A side surface of the flexible display module, a side surface of the flexible functional module, and a side surface of the flexible window module may be located on a first plane at a first side portion. An inner angle formed by an upper surface of the flexible window module emitting light and the first plane may be an acute angle at the first side portion.