Disclosed are semiconductor devices and methods of fabricating the same. The method comprises forming on a substrate a mold structure including a plurality of sacrificial patterns and a plurality of dielectric patterns that are alternately stacked, patterning the mold structure to form a plurality of preliminary stack structures extending in a first direction, forming on the preliminary stack structures a support pattern that extends in a direction intersecting the first direction and extends across the preliminary stack structures, and replacing the sacrificial patterns with conductive patterns to form a plurality of stack structures from the preliminary stack structures. The support pattern remains on the stack structures.