Semiconductor devices and methods of forming the same are provided. Semiconductor devices may include first and second active patterns on a substrate. Each of the first and second active patterns may extend in a first direction. The first and second active patterns may be aligned along the first direction and may be separated by a first trench extending in a second direction. The first trench may define a first sidewall of the first active pattern. The semiconductor devices may also include a channel pattern including first and second semiconductor patterns stacked on the first active pattern, a dummy gate electrode on the channel pattern and extending in the second direction, and a gate spacer on one side of the dummy gate electrode, the one side of the dummy gate electrode being adjacent to the first trench. The gate spacer may cover a first sidewall of the first active pattern.