A semiconductor device includes a substrate including a first device region and a second device region, a first active pattern on the first device region, a second active pattern, which has a width smaller than the first active pattern, on the second device region, a first channel pattern on the first active pattern, a first source/drain pattern connected to the first channel pattern, a second channel pattern on the second active pattern, a second source/drain pattern connected to the second channel pattern, and a gate electrode that extends from the first channel pattern to the second channel pattern in a first direction. The first channel pattern includes a plurality of semiconductor patterns, which are vertically stacked and spaced apart from each other. The second channel pattern protrudes vertically from the second active pattern.