A three-dimensional semiconductor device comprises a stack structure on a lower structure, a vertical channel structure passing through the stack structure, and a first vertical support structure passing through the stack structure and spaced apart from the vertical channel structure. The stack structure includes interlayer insulation layers and gate horizontal patterns, alternately stacked in a vertical direction perpendicular to an upper surface of the lower structure. The vertical channel structure and the first vertical support structure have different cross-sectional shapes. The vertical channel structure further includes a channel semiconductor layer. The vertical channel structure includes first and second vertical regions, and a width variation portion between the first and second vertical regions. The interlayer insulation layers include an intermediate interlayer insulation layer adjacent to the width variation portion. The intermediate interlayer insulation layer has the same thickness as that of an interlayer insulation layer adjacent in the vertical direction.