A semiconductor device includes a substrate including an active region extending in a first direction, a gate structure intersecting the active region on the substrate and extending in a second direction, where the active region includes a recessed region at at least one side of the gate structure, a plurality of channel layers on the active region, spaced apart from each other in a third direction that is substantially perpendicular to an upper surface of the substrate, and at least partially surrounded by the gate structure and a source/drain region in the recessed region of the active region and connected to the plurality of channel layers.