A foldable display device includes a flexible display panel including a first plane region, a second plane region, and a folding region positioned between the first plane region and the second plane region, a window on the flexible display panel, the window including an excess region that does not overlap the flexible display panel in at least one plane region direction of the first plane region and the second plane region based on the folding region while covering the flexible display panel, and a deformation preventing sheet overlapping the excess region of the window.