A semiconductor device may include a substrate, an electrode structure including electrodes stacked on the substrate, an upper semiconductor pattern penetrating at least a portion of the electrode structure, and a lower semiconductor pattern between the substrate and the upper semiconductor pattern. The upper semiconductor pattern includes a gap-filling portion and a sidewall portion extending from the gap-filling portion in a direction away from the substrate, the lower semiconductor pattern includes a concave top surface, the gap-filling portion fills a region enclosed by the concave top surface, a top surface of the gap-filling portion has a rounded shape that is deformed toward the substrate, and a thickness of the sidewall portion is less than a thickness of the gap-filling portion.