A memory device includes: a memory cell region including gate electrodes spaced apart from each other on a first semiconductor substrate to be stacked, and channel structures; and a peripheral circuit region including upper metal lines disposed above a second semiconductor substrate, disposed below the memory cell region. The first semiconductor substrate includes first regions, having a first value corresponding to a distance between the first semiconductor substrate and the upper metal lines, and second regions having a second value, lower than the first value. A reference voltage for operating the memory device is transmitted to at least one of the first upper metal lines, disposed below the first region. Accordingly, coupling capacitance for a significant signal may be reduced while maintaining a length of a connection portion and the magnitude of resistance of a common source line. Furthermore, an error rate of the memory device may be reduced.