A semiconductor device includes active regions on a substrate, a gate structure intersecting the active regions, a source/drain region on the active regions and at a side surface of the gate structure, a gate spacer between the gate structure and the source/drain region, the gate spacer contacting the side surface of the gate structure, a lower source/drain contact plug connected to the source/drain region, a gate isolation layer on the gate spacer, an upper end of the gate isolation layer being at a higher level than an upper surface of the gate structure and an upper surface of the lower source/drain contact plug, a capping layer covering the gate structure, the lower source/drain contact plug, and the gate isolation layer, and an upper source/drain contact plug connected to the lower source/drain contact plug and extending through the capping layer.