A semiconductor device includes a substrate including an active region in a first direction, a plurality of channel layers on the active region and disposed in a direction perpendicular to an upper surface of the substrate, a gate electrode respectively surrounding the plurality of channel layers, and a source/drain structure respectively disposed on both sides of the gate electrode in the first direction and connected to each of the plurality of channel layers. The gate electrode extends in a second direction crossing the first direction. The gate electrode includes an overlapped portion in a region of the gate electrode on an uppermost channel layer of the plurality of channel layers. The overlapped portion of the gate electrode overlaps the source/drain structure in the first direction and has a side surface inclined toward the upper surface of the substrate.