A semiconductor device includes: an active region extending on a substrate in a first direction; a plurality of semiconductor layers spaced apart from each other vertically on the active region, including a lower semiconductor layer and an uppermost semiconductor layer disposed above the lower semiconductor layer and having a thickness greater than that of the lower semiconductor layer; a gate structure extending on the substrate in a second direction, perpendicular to the first direction, and including a gate electrode at least partially surrounding each of the plurality of semiconductor layers; a spacer structure disposed on both sidewalls of the gate structure; and source/drain regions disposed on the active region on both sides of the gate structure and contacting the plurality of semiconductor layers.