Disclosed is a three-dimensional semiconductor memory device including a substrate including a cell array region and a connection region, a stack including first and second stacks sequentially stacked on the substrate, the stack having a staircase structure on the connection region, each of the first and second stacks including conductive patterns vertically stacked on the substrate, and contact plugs disposed on the connection region and respectively coupled to the conductive patterns. A bottom surface of each contact plug is located between top and bottom surfaces of a corresponding conductive pattern. In each stack, a recess depth of each contact plug varies monotonically in a stacking direction of the conductive patterns, when measured from a top surface of a corresponding conductive pattern. The contact plugs coupled to an uppermost conductive pattern of the first stack and a lowermost conductive pattern of the second stack have discrete recess depths.