A three-dimensional semiconductor memory device includes; a first block and a second block arranged on a first substrate in a first direction, wherein each of the first block and the second block includes electrode layers stacked on the first substrate, a source layer interposed between the first block and the first substrate, and between the second block and the first substrate, a first insulating separation pattern interposed between the first block and the second block and extending in the first direction, wherein the first insulating separation pattern includes a line portion and a protruding portion, the line portion extending in a second direction crossing the first direction, and the protruding portion having a width greater than a width of the line portion, a first source contact plug penetrating the protruding portion of the first insulating separation pattern to electrically connect the source layer, and at least one through via penetrating the source layer and at least one of the first block and the second block.