A semiconductor device includes: a first active pattern on a substrate and including a first active fin and a second active fin; a device isolation layer defining the first active pattern; a gate electrode crossing the first active pattern; a first source/drain pattern and a second source/drain pattern on the first active fin and the second active fin, respectively; an inner fin spacer between the first and second source/drain patterns; and a buffer layer between the first and second active fins, wherein the inner fin spacer includes: a first inner spacer portion contacting the first source/drain pattern; a second inner spacer portion contacting the second source/drain pattern; and an inner extended portion extending from the first and second inner spacer portions, wherein the inner extended portion is between the first and second active fins, wherein the buffer layer has a dielectric constant higher than that of the inner fin spacer.