A semiconductor device includes a substrate, an active pattern extending in a first direction on the substrate, first and second nanosheets stacked on the active pattern to be spaced apart from each other in a vertical direction, a gate electrode that extends in a second direction the active pattern, the gate electrode surrounding each of the first and second nanosheets, a source/drain region on at least one side of the gate electrode, and inner spacers between the gate electrode and the source/drain region, the inner spacers including a first inner spacer between the active pattern and the first nanosheet, and a second inner spacer between the first nanosheet and the second nanosheet, the second inner spacer having a first portion adjacent to the first nanosheet, and a second portion adjacent to the second nanosheet, the first portion being wider than the second portion.