A 3D semiconductor memory device includes a peripheral circuit structure including a first row decoder region, a second row decoder region, and a control circuit region between the first and second row decoder regions, a first electrode structure and a second electrode structure on the peripheral circuit structure, spaced apart in a first direction, and each including stacked electrodes, a mold structure on the peripheral circuit structure between the first and second electrode structures and including stacked sacrificial layers, vertical channel structures penetrating the first and second electrode structures, a separation insulating pattern provided between the first electrode structure and the mold structure and penetrating the mold structure, and a separation structure intersecting the first electrode structure in the first direction and extending to the separation insulating pattern, wherein a maximum width of the separation insulating pattern in a second direction is greater than a maximum width of the separation structure in the second direction.