A semiconductor device includes a lower insulating layer on a lower substrate, a lower pad structure inside the lower insulating layer, an upper insulating layer on the lower insulating layer, an upper pad structure inside the upper insulating layer, and an upper substrate on the upper insulating layer. A via plug passes through at least a portion of each of the upper substrate, the upper insulating layer, and the lower insulating layer, and in contact with the upper pad structure and the lower pad structure. The upper pad structure includes upper pad conductive layers and an upper connection layer between the upper pad conductive layers. The upper connection layer includes a conductive pattern having a shape different from a shape of at least one of the upper pad conductive layers. The via plug is in direct contact with the upper pad conductive layers and the upper connection layer.