A semiconductor device includes a lower stack structure including lower gate electrodes and lower insulating layers that are alternately and repeatedly stacked on a substrate. The semiconductor device includes an upper stack structure including upper gate electrodes and upper insulating layers that are alternately and repeatedly stacked on the lower stack structure. A lower channel structure penetrates the lower stack structure. An upper channel structure penetrates and is connected to the upper stack structure. A lower vertical insulator is disposed between the lower stack structure and the lower channel structure. The lower channel structure includes a first vertical semiconductor pattern connected to the substrate, and a first connecting semiconductor pattern disposed on the first vertical semiconductor pattern. The upper channel structure includes a second vertical semiconductor pattern electrically connected to the first vertical semiconductor pattern with the first connecting semiconductor pattern disposed therebetween.