A semiconductor device including an active pattern extending in a first direction; a channel pattern on the active pattern and including vertically stacked semiconductor patterns; a source/drain pattern in a recess in the active pattern; a gate electrode on the active pattern and extending in a second direction crossing the first direction, the gate electrode surrounding a top surface, at least one side surface, and a bottom surface of each of the semiconductor patterns; and a gate spacer covering a side surface of the gate electrode and having an opening to the semiconductor patterns, wherein the source/drain pattern includes a buffer layer covering inner sides of the recess, the buffer layer includes an outer side surface and an inner side surface, which are opposite to each other, and each of the outer and inner side surfaces is a curved surface that is convexly curved toward a closest gate electrode.