A three-dimensional (3D) semiconductor device includes a stack of conductive layers spaced from each other in a vertical direction, the stack having a staircase-shaped section in a connection region, and ends of the conductive layers constituting treads of the staircase-shaped section, respectively. The 3D semiconductor device further includes buffer patterns disposed on and protruding above the respective ends of the conductive layers, an interconnection structure disposed above the stack and including conductive lines, and contact plugs extending vertically between the conductive lines and the buffer patterns and electrically connected to the conductive layers of the stack via the buffer patterns.