Semiconductor devices are provided. A semiconductor device includes a substrate. The semiconductor device includes a stack structure including conductive layers stacked on the substrate. Moreover, the semiconductor device includes a dummy structure penetrating a stepped region of the stack structure. A portion of the dummy structure includes a first segment and a second segment. The first segment extends in a first direction in a plane parallel to an upper surface of the substrate. The second segment protrudes from the first segment in a second direction, in the plane, that intersects the first direction.