A semiconductor device includes a peripheral circuit region comprising a first substrate, circuit elements on the first substrate, a first insulating layer covering the circuit elements, and a contact plug passing through the first insulating layer and disposed to be connected to the first substrate; and a memory cell region comprising a second substrate, gate electrodes on the second substrate and stacked in a vertical direction, and channel structures passing through the gate electrodes, wherein the contact plug comprises a metal silicide layer disposed to contact the first substrate and having a first thickness, a first metal nitride layer on the metal silicide layer to contact the metal silicide layer and having a second thickness, greater than the first thickness, a second metal nitride layer on the first metal nitride layer, and a conductive layer on the second metal nitride layer.