A semiconductor memory device includes a first memory cell provided on a substrate, a second memory cell provided on the substrate and spaced apart from the first memory cell, a passivation layer extending along a side surface of the first memory cell and a side surface of the second memory cell, and a gap fill layer covering the passivation layer. Each of the first memory cell and the second memory cell includes a selection pattern having ovonic threshold switching characteristics, and a storage pattern provided on the selection pattern. The passivation layer includes a lower portion filling a space between the selection pattern of the first memory cell and the selection pattern of the second memory cell, and an upper portion extending along a side surface of the storage pattern of each of the first memory cell and the second memory cell.