Wiring structures of three-dimensional semiconductor devices and methods of forming the same are provided. The wiring structures may include an upper wordline and a lower wordline, each of which extends in a longitudinal direction. The upper wordline may include a recessed portion that extends for only a portion of the upper wordline in a transverse direction and the lower wordline may include a wiring area exposed by the recessed portion of the upper wordline. The wiring structures may also include an upper contact plug contacting the upper wordline and a lower contact plug contacting the wiring area. The upper and lower contact plugs may extend in a vertical direction.