A semiconductor device includes a first stacked structure and a second stacked structure spaced apart from each other on a substrate, and a plurality of separation structures and a plurality of vertical memory structures alternately arranged between the first stacked structure and the second stacked structure in a first direction parallel to an upper surface of the substrate. Each of the first and second stacked structures includes a plurality of interlayer insulating layers and a plurality of gate layers alternately repeatedly stacked on the lower structure. Each of the vertical memory structures includes a first data storage structure facing the first stacked structure and a second data storage structure facing the second stacked structure. Side surfaces of the first and second stacked structures facing the vertical memory structures are concave in a plan view.