A semiconductor device including a gate separation region is provided. The semiconductor device includes an isolation region between active regions; interlayer insulating layers on the isolation region; gate line structures overlapping the active regions, disposed on the isolation region, and having end portions facing each other; and a gate separation region disposed on the isolation region, and disposed between the end portions of the gate line structures facing each other and between the interlayer insulating layers. The gate separation region comprises a gap fill layer and a buffer structure, the buffer structure includes a buffer liner disposed between the gap fill layer and the isolation region, between the end portions of the gate line structures facing each other and side surfaces of the gap fill layer, and between the interlayer insulating layers and the side surfaces of the gap fill layer.