A semiconductor device includes a first active region on a substrate, channel layers disposed on the first active region to be spaced apart from each other in a vertical direction, a first gate structure disposed on the first active region and surrounding each channel layer, and a first source/drain region on the first active region on at least one side of the first gate structure. The channel layers include first to third channel layers. The first gate structure includes a first gate electrode and a first gate dielectric layer. The first gate dielectric layer includes first to third portions surrounding the first to third channel layers, respectively. The second portion has a thickness greater than a thickness of the first portion, and the third portion has a thickness greater than the thickness of the second portion.