A semiconductor device includes a first semiconductor structure including a first substrate, circuit devices on the first substrate, a lower interconnection structure on the circuit devices, and a lower bonding structure electrically connected to the lower interconnection structure, and a second semiconductor structure including a second substrate disposed on the first semiconductor structure, gate electrodes stacked and spaced apart from each other in a first direction perpendicular to a lower surface of the second substrate, channel structures that penetrate the gate electrodes and extend in the first direction, and an upper bonding structure electrically connected to the gate electrodes and the channel structures and bonded to the lower bonding structure. The second semiconductor structure further includes a first via connected to an upper portion of the second substrate, a second via spaced apart from the first via and the second substrate, and a contact plug.