A foldable display device includes a display panel including a folding area and non-folding areas on both sides of the folding area and a back plate disposed under the display panel; and a support substrate disposed under the back plate and made of a metal or alloy including a plurality of pores, the support substrate includes a plurality of groove patterns recessed from a bottom surface toward a top surface of the support substrate and corresponding to the folding area, thereby minimizing recognition of the patterns in the folding area of the display panel and improve the folding properties.