A three-dimensional semiconductor memory device may include a peripheral circuit structure including transistors on a first substrate, and a cell array structure on the peripheral circuit structure, the cell array structure including: a first stack structure block comprising first stack structures arranged side by side in a first direction on a second substrate, a second stack structure block comprising second stack structures arranged side by side in the first direction on the second substrate, a separation structure disposed on the second substrate between the first stack structure block and the second stack structure block and comprising first mold layers and second mold layers, and a contact plug penetrating the separation structure. The cell array structure may include a first metal pad and the peripheral circuit structure may include a second metal pad. The first metal pad may be in contact with the second metal pad.