A semiconductor device includes a first trench on the device region, a first device isolation layer in the first trench and defining an active pattern of the device region, a second trench on the interface region, and a second device isolation layer in the second trench. The second isolation layer includes a buried dielectric pattern, a dielectric liner pattern on the buried dielectric pattern, and a first gap-fill dielectric pattern on the dielectric liner pattern. The buried dielectric pattern includes a floor segment on a floor of the second trench, and a sidewall segment on a sidewall of the second trench. The sidewall segment has a thickness different from a thickness of the floor segment.