Semiconductor devices include a substrate having a target connection region; a conductive line having a first side wall spaced apart from the substrate by at least an insulating layer, and a conductive plug structure electrically connecting the conductive line to the target connection region, wherein the conductive plug includes a first conductive plug having a first side wall, a bottom surface contacting the target connection region of the substrate, and a second side wall facing the first side wall of the conductive line, and a second conductive plug between the conductive line and the first conductive plug. The second conductive plug contacts both the first side wall of the conductive line and the second side wall of the first conductive plug.