A semiconductor device includes a peripheral circuit region with a first substrate, circuit devices on the first substrate, and a first wiring structure, a memory cell region with a second substrate that has a first region and a second region, gate electrodes stacked in the first region, channel structures that penetrate the gate electrodes, a first horizontal conductive layer on the second substrate in the first region, an insulating region on the second substrate in the second region, a second horizontal conductive layer on the first horizontal conductive layer and the insulating region, and a second wiring structure, and a third wiring structure that connects the first substrate to the second substrate, and includes an upper via that penetrates the second horizontal conductive layer, the insulating region, and the second substrate, and a lower wiring structure connected to the upper.